Rails 4 upgrade

I've made a few modifications to https://github.com/rubygems/rubygems.org/tree/rails4 and brought everything else up to date. It looks like its all working but let me know if there are any specific parts of the application that you'd like an extra pair of eyes on.

Rails 4 now prefixes jsonp with comments (see #bbc8d10) which I don't think will cause us any issues, but may cause issues for any third party services using our API.

Personally, I'd prefer copying in the full default rails config for each env (changed for our config where necessary). That will make it easier to update as rails updates.