Ruby autogenerated files should require 'thrift' gem

The gen-rb files currently don't require the thrift gem. This means that the user of such a file must explicitly require 'thrift' in his code before requiring a gen-rb file, which can be a bit annoying (especially when debugging / experimenting in IRB). I propose that we change the compiler to add the require at the top of every gen-rb file.

Patch coming shortly.