Upgrade to ember-data 13 and ember.js RC4/5

emberjs/data@204fd3046e9c19b2e7f48f8a929e98df4a00e5b5

has changed some semantics of the ajax call, breaking some methods

trying to delete a record will cause an error in .ajax about hash being undefined.

the ajax call in their rest adapter has changed to return a promise and accept an undefined hash.

the changes need to be made here as well I suppose.

Although, looking at the override here, I don't see why an override is needed at all.  The default in DS.RESTAdapter seems like it would work fine.

Testing it with the function removed I see no issues, so I propose the ajax override be removed from here.