Enable testing against multiple Ember versions.
Enable testing against multiple Ember versions.

If `Ember::Source` is defined and we are testing against the `release`
channel, use the local version of Ember from the `ember_source` gem.
Otherwise, use the published builds from S3 for the proper channel.

emberjs/ember-dev,mmun/ember-dev,emberjs/ember-dev,mmun/ember-dev