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.
Merge pull request #428 from rjackson/update_ember

Update to Ember 1.4.0-beta.2