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.
Conflicts:
	packages/ember-testing/tests/acceptance_test.js
	packages/rsvp/lib/main.js