[BUGFIX beta] Handle beta branch builds properly.

The initial implementation of the `FORCE_BRANCH` flag did not properly account
for all scenarios.  This attempts to fix an issue with builds of commits
pushed directly to the beta/stable branches.

As discussed with @wagenet in IRC.

The main changes were implemented in `ember-dev` in these PRs:

* https://github.com/emberjs/ember-dev/pull/112
* https://github.com/emberjs/ember-dev/pull/113