gistfile1.rb

Preference-ENGINE:/Users/tomohiro:(---) 2.0.0-rc2
2:16:> irb
irb(main):001:0> require 'fiddle'
=> true
irb(main):002:0> DL
NameError: uninitialized constant DL
  from (irb):2
	from /Users/tomohiro/.rbenv/versions/2.0.0-rc2/bin/irb:12:in `<main>'
irb(main):003:0> 

gistfile2.rb

Preference-ENGINE:/Users/tomohiro:(---) 2.0.0-rc2
2:18:> irb
irb(main):001:0> require 'dl'
DL is deprecated, please use Fiddle
=> true
irb(main):002:0> DL
=> DL
irb(main):003:0> DL.fiddle?
=> true
irb(main):004:0> 

gistfile3.rb

Preference-ENGINE:/Users/tomohiro:(---) 1.9.3-p374
2:18:> irb
irb(main):001:0> require 'fiddle'
=> true
irb(main):002:0> DL
=> DL
irb(main):003:0> DL.fiddle?
=> true
irb(main):004:0> 

Sorry, it's not convenient, but you need to conditionalize:

diff --git a/lib/spring/sid.rb b/lib/spring/sid.rb
index 632c694..c70e212 100644
--- a/lib/spring/sid.rb
+++ b/lib/spring/sid.rb
@@ -2,8 +2,14 @@ require 'fiddle'

 module Spring
   module SID
+    if RUBY_VERSION >= '2.0.0'
+      handle = Fiddle::Handle
+    else
+      handle = DL::Handle
+    end
+
     FUNC = Fiddle::Function.new(
-      DL::Handle::DEFAULT['getsid'],
+      handle::DEFAULT['getsid'],
       [Fiddle::TYPE_INT],
       Fiddle::TYPE_INT
     )

I'll send a patch.

@tenderlove Thank you for your information!
and thank you for your patch for spring!

done: rails/spring#51