Update to latest RSVP (1fae8bd).

Update to the lastest version of RSVP. See changes
[here](https://github.com/tildeio/rsvp.js/compare/ae4abbccea971a7a7a0437d8275b6c5f1f21ff4c...1fae8bdad791c1d6125223433ce3b45b7954e1fd).

I can change this to use `[BUGFIX beta]`. If we do
not want to update the beta branch, we should merge #3743 first.

As requested by @stefanpenner in
https://github.com/emberjs/ember.js/pull/3743#issuecomment-29085092.