Use a Railtie to avoid deprecation warnings in rails 4.2

Rails 4.2 has a deprecation warning upon calling `after_commit`. In normal circumstances this is avoided by a configuration. However due to the direct injection of the after commit action module upon require `after_commit` is called /before/ those configurations can apply, thus permanently causing deprecation warnings.

This is easily solved using a Railtie to inject the module at a more appropriate time.