Fixed bug with native gem generation.

Task was inserting 'compile:mri' into gem_spec.files, causing rubygems to complain that the file wasn't found.  Fixed by using ext_name instead of task prerequisites.

tasks/ext_helper.rb

@@ -58,7 +58,7 @@ def setup_c_extension(extension_name, gem_spec = nil)
 
         # add the precompiled binaries to the list of files
         # (taken from compile task dependency)
-        gem_spec.files += Rake::Task['compile'].prerequisites
+        gem_spec.files += ["lib/#{ext_name}"]
       end
     end
   end