Bug 1091066 - Continuous Integration in travis-ci for b2g-manifest

After this is merged in, someone with admin access should be enable travis testing from:
https://travis-ci.org/profile/mozilla-b2g

From this point on, it would make sense only to merge pull requests if the user's fork has also passed the tests in travis (each user will need to enable travis testing for their own fork).

This should avoid bustage from b2g-manifest hitting gecko and causing tree closures.

The new CI checks that b2g bumper can run successfully against the b2g-manifest manifests.