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.

There are more configs in Rails 4's development.rb, do we need to add these?

```ruby
  config.active_support.deprecation = :log
  config.active_record.migration_error = :page_load
  config.assets.debug = true
  config.assets.raise_runtime_errors = true
```