Extensions are not compiling correctly

I ran into a problem compiling a proto file that imports and extends another file.  I was able to replicate this behavior with the addressbook_ext.proto in this repo's source.

To replicate:

    cd ~/.rbenv/versions/1.9.3-p327/lib/ruby/gems/1.9.1/gems/protobuf-2.6.4
    mkdir tmp
    rprotoc --ruby_out=tmp test/proto/addressbook*.proto 
    diff test/proto/addressbook_ext.pb.rb tmp/test/proto/addressbook_ext.pb.rb 

This [gist](https://gist.github.com/zroger/4746387) shows the output of the generated test file vs the one that ships with the repo.