Rails 4.2 edge

A fix was committed to Rails master recently that fixes an issue in our application where the `updated_at` timestamp would get updated on all custom levels even when they haven't been modified (see [level_test.rb:153](https://github.com/code-dot-org/dashboard/compare/rails-4.2-edge#diff-9fa389a86852e954173578394e23c2fbR153) for the test). Upgrading to Rails 4.2.0.alpha fixes this issue. Several slight modifications to the application were necessary to get all tests passing with the new Rails version.