Register engine with app isntead of Sprockets

I'm running a Rails 4 app that doesn't work properly with emblem-rails, and I've identified the issue. With the original code, registering the engine with `Sprockets` directly doesn't pick up any `.emblem` files. By registering the engine with `app.assets`, it works properly.

Note that [ember-rails uses app.assets](https://github.com/emberjs/ember-rails/blob/master/lib/ember_rails.rb#L33-L41), so I figured this change should be safe.