Allow usage of Ember >= 1.5.0-beta.4.

Ember changed the listeners that were used by ember-testing in
1.5.0-beta.4 and higher.

This change allows both old and new versions to be unbound as
appropriate.

Reference:

* https://github.com/emberjs/ember.js/pull/4421
Merge pull request #12 from rjackson/make-work-with-newer-ajax-events

Allow usage of Ember >= 1.5.0-beta.4.