Upgrading to Rails 4.2.0.beta1

In preparation for rails 4.2, I've upgraded to the beta.

Things of note:

* Using the responders gem for now
* Rails intends to [raise errors inline from `after_commit` hooks](https://github.com/rails/rails/pull/16537), so I've opted in for that behaviour now
* Application setup now done via `bin/setup`
* Dropped figaro and application.yml. If we need anything like this in the future we'll use [`Rails.application.config_for`](https://github.com/rails/rails/pull/16129)