Remove `RSVP.Promise#fail` in favor of `catch`.


After discussion with @wycats and @domenic, it was decided we should remove
`fail` before it enters an official release

- es3 users should use the ['catch'] syntax
- plans are to add a transpiler for people to es5 -> es3 (for finally and catch) etc.
Merge pull request #3902 from stefanpenner/remove-promise-fail

Remove `RSVP.Promise#fail` in favor of `catch`.