Update to Ember 1.4.0-beta.2

Ember 1.3.1 has an issue that prevents loading/error substates from working properly (see [detailed comment](https://github.com/stefanpenner/ember-jj-abrams-resolver/issues/13#issuecomment-32726797)).

This was fixed upstream and is working properly on Ember 1.4.0-beta.2.