@macbellingrath pointed out that Xcode 10 had a change and line 80 needed to be updated.

post-archive-script.sh

#################################################################################################################################################
# post-archive-script.sh
#
# The purpose of this script is to create a universal binary for your framework
# Also - if there is a problem with steps in this script, then it should be
# easy to debug!  Other scripts that this is based off of aren't so easy to debug.
#
#################################################################################################################################################


# Validate that everything is setup correctly
validate() {
  
  # 1. Make sure the archival is coming from a workspace (not a project).
  # This is because a project doesn't provide enough enviornment variables to
  # correctly archive the project in the way we need to (yes, lazy programmer).
  if [ "${PROJECT_NAME}" == "" ]
  then
    exec > /tmp/${SCHEME_NAME}_archive.log 2>&1;
    echo "[ERROR]: PROJECT_NAME was not defined, did you select the 'Provide build settings from' in the 'Post-actions' for Archival?";
    envExit 1;
    
  else
    exec > /tmp/${IBSC_MODULE}_archive.log 2>&1;
  fi
  
  # Make sure the project is setup to actually archive, otherwise we don't do anything.
  if [ "${ARCHIVE_PRODUCTS_PATH}" == "" ] ; then
    echo "[ERROR]: ARCHIVE_PRODUCTS_PATH is not defined - you probably need to set the SKIP_INSTALL build setting to NO"
    envExit 1;
  else
    echo "ARCHIVE_PRODUCTS_PATH=${ARCHIVE_PRODUCTS_PATH}"
  fi
}

# Configure some environment variables and ensure the universal folder exists
configure() {
  UNIVERSAL_OUTPUTFOLDER=${BUILD_DIR}/${CONFIGURATION}-universal
  if [ ! -d "${UNIVERSAL_OUTPUTFOLDER}" ]
  then
    mkdir -p "${UNIVERSAL_OUTPUTFOLDER}"
  fi
  PROJECT_FOLDER=$(dirname "${PROJECT_FILE_PATH}");
  
  if [ "${CONFIGURATION}" == "" ]
  then
    echo "CONFIGURATION was not defined, setting it to Release"
    CONFIGURATION=Release
  fi
}

# If we're going to non-zero exit, lets print out the environment first (again,
# for debugging purposes, this is very useful).
envExit() {
  env;
  exit $@;
}

# The work is done here, if we haven't been invoked yet, then go through the
# build steps, and dump the output to the log file.  You'll note that there are
# no comments in this function.  That's because the echo statements serve as
# those comments and provide breadcrumbs in the case that we can't build
buildFramework() {
  
  if [ "true" == ${ALREADYINVOKED:-false} ]
  then
    echo "RECURSION: Detected, stopping"
  else
    export ALREADYINVOKED="true"
    
    echo "Step 1. Building for iPhoneSimulator"
    xcodebuild -workspace "${WORKSPACE_PATH}" -scheme "${SCHEME_NAME}" -configuration ${CONFIGURATION} -sdk iphonesimulator -destination 'platform=iOS Simulator,name=iPhone 6' ONLY_ACTIVE_ARCH=NO ARCHS='i386 x86_64' BUILD_DIR="${BUILD_DIR}" BUILD_ROOT="${BUILD_ROOT}" ENABLE_BITCODE=YES OTHER_CFLAGS="-fembed-bitcode" BITCODE_GENERATION_MODE=bitcode clean build
    if [ "$?" != "0" ]
    then
      echo "[ERROR]: FAILED Step 1: Building for iPhoneSimulator";
      envExit 1
    fi
    
    echo "Step 2. Copy the framework structure (from iphoneos build) to the universal folder"
    cp -R "${ARCHIVE_PRODUCTS_PATH}${INSTALL_PATH}" "${UNIVERSAL_OUTPUTFOLDER}/"
    if [ "$?" != "0" ]
    then
      echo "[ERROR]: FAILED Step 2: Copy the framework structure (from iphoneos build) to the universal folder";
      envExit 1
    fi
    
    echo "Step 3. Copy Swift modules from iphonesimulator build (if it exists) to the copied framework directory"
    SIMULATOR_SWIFT_MODULES_DIR="${BUILD_DIR}/${CONFIGURATION}-iphonesimulator/${PROJECT_NAME}.framework/Modules/${PROJECT_NAME}.swiftmodule/."
    if [ -d "${SIMULATOR_SWIFT_MODULES_DIR}" ]; then
      cp -R "${SIMULATOR_SWIFT_MODULES_DIR}" "${UNIVERSAL_OUTPUTFOLDER}/${PROJECT_NAME}.framework/Modules/${PROJECT_NAME}.swiftmodule"
      if [ "$?" != "0" ]
      then
        echo "[ERROR]: FAILED Step 3: Copy Swift modules from iphonesimulator build (if it exists) to the copied framework directory";
        envExit 1
      fi
    fi
    
    echo "Step 4. Create universal binary file using lipo and place the combined executable in the copied framework directory"
    lipo -create -output "${UNIVERSAL_OUTPUTFOLDER}/${EXECUTABLE_PATH}" "${BUILD_DIR}/${CONFIGURATION}-iphonesimulator/${EXECUTABLE_PATH}" "${ARCHIVE_PRODUCTS_PATH}${INSTALL_PATH}/${EXECUTABLE_PATH}"
    if [ "$?" != "0" ]
    then
      echo "[ERROR]: FAILED Step 4: Create universal binary file using lipo and place the combined executable in the copied framework directory";
      envExit 1
    fi
    
    echo "Step 5. Create universal binaries for embedded frameworks";
    FRAMEWORKS_DIR="${UNIVERSAL_OUTPUTFOLDER}/${PROJECT_NAME}.framework/Frameworks";
    if [ -e "${FRAMEWORKS_DIR}" ]
    then
      for SUB_FRAMEWORK in $(ls "${FRAMEWORKS_DIR}")
      do
        BINARY_NAME="${SUB_FRAMEWORK%.*}"
        lipo -create -output "${UNIVERSAL_OUTPUTFOLDER}/${PROJECT_NAME}.framework/Frameworks/${SUB_FRAMEWORK}/${BINARY_NAME}" "${BUILD_DIR}/${CONFIGURATION}-iphonesimulator/${SUB_FRAMEWORK}/${BINARY_NAME}" "${ARCHIVE_PRODUCTS_PATH}${INSTALL_PATH}/${PROJECT_NAME}.framework/Frameworks/${SUB_FRAMEWORK}/${BINARY_NAME}"
        if [ "$?" != "0" ]
        then
          echo "[ERROR]: FAILED Step 5 (${SUB_FRAMEWORK}): Create universal binaries for embedded frameworks";
          envExit 1
        fi
      done
    fi
    
    echo "Step 6. Convenience step to copy the framework to the project's directory"
    yes | cp -Rf "${UNIVERSAL_OUTPUTFOLDER}/${FULL_PRODUCT_NAME}" "${ARCHIVE_PRODUCTS_PATH}${INSTALL_PATH}" && \
    yes | cp -Rf "${UNIVERSAL_OUTPUTFOLDER}/${FULL_PRODUCT_NAME}" "${PROJECT_FOLDER}"
    if [ "$?" != "0" ]
    then
      echo "[ERROR]: FAILED Step 6: Convenience step to copy the framework to the project's directory";
      envExit 1;
    fi
    
  fi
}

archiveFramework() {
  
  echo "Step 7. Create a zip archive of the framework"
  if [ ! -e "${PROJECT_FOLDER}/${IBSC_MODULE}.framework" ]
  then
    echo "[ERROR]: FAILED Step 7: ${IBSC_MODULE}/${PROJECT_NAME}.framework doesn't exist";
    envExit 1;
  fi
  
  cd "${PROJECT_FOLDER}"
  VERSION=$(grep s.version ../${PRODUCT_NAME}.podspec|grep -v s.source|grep -v cocoapods_version|sed 's/s.version//'|sed 's/[^\.,0-9]//g');
  zip -r "${IBSC_MODULE}-${VERSION}.zip" "${IBSC_MODULE}.framework"
}

# main function - delegates to the helpers
main() {
  validate;
  configure;
  buildFramework;
  archiveFramework;
  
  open "${PROJECT_FOLDER}";
}

main;

Running

To use this, you either copy / paste the contents of this script into the Post-actions of your framework scheme, or drop this as an executable script file somewhere (~/bin/post-archive-script.sh in this example) and call it from your Post-actions run script. Here is a screenshot:

After archiving, it will open up the folder (should be your project folder) with the finder and you should see your universal framework.

Reference

• Publish a Universal Binary iOS Framework in Swift using CocoaPods
  

If you want to build the framework from fastlane, you can do something like this:

  desc "Builds the Framework"
  lane :framework do
    gym(
      scheme: "Framework",
      include_bitcode: true,
      clean: true,
      export_method: 'package',
      skip_package_ipa: true
    )
  end

Thanks to @macbellingrath for a change required to support Xcode 10