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.

Looks like it was removed by @masonforest  in https://github.com/georgedrummond/rubygems.org/commit/52a2a35a932d4a4f4216bfc179c171e93287c954#diff-ee96e2d5cc5ac47f847e19de030867cdL201

I don't think activerecord allows you to cast an array when it expects a string and it raises an exception before validations can even run.

How should I handle this behaviour?