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 clearance already has a route named ```sign_out``` that can't just be overridden. Rails 4 makes this more complicated since we'd have to disable clearance routing and copy all the default routes (minus this one) into our routes file. https://github.com/thoughtbot/clearance#overriding-routes