Update deferred docs to not use `new`

Based on this PR: https://github.com/emberjs/ember.js/pull/2571 we don't need to call RSVP.defer() with new. This section of the readme is confusing because it clashes with the Ember docs' defer example: http://emberjs.com/api/classes/Ember.RSVP.html#method_defer
Merge pull request #210 from nikolas/patch-1

Update deferred docs to not use `new`