Use ember.debug.js if it exist

`ember.js` is deprecated in favor of `ember.debug.js` later than Ember.js 1.10.0.beta.1.
* https://github.com/emberjs/ember.js/issues/9830
* https://github.com/emberjs/ember.js/pull/9845

The test for this feature couldn’t be added before ember-source 1.10.0.beta.1 released.
Should I wait new release?