Update to latest RSVP (80cec268).

A few of the changes:

* Added `Promise.cast`
* Added `Promise.finally`
* Added `RSVP.on('error', callback)`.

See full changes [here](https://github.com/tildeio/rsvp.js/compare/ae4abbccea971a7a7a0437d8275b6c5f1f21ff4c...80cec268a5ef91273b2dd6cd83c3d3d4f4d427df).

Once this is merged, I will update #3743 to use the new `RSVP.on('error', …)` syntax.