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.

On a CI-Server calling it manually should be fine. It'll ensure the database schema is in a consistent state even if you modify migrations along the way.

I'll backport the patch to `4.1.` in the near future.