force a failure if you try and release a branch that is not in staging

This should help us prevent accidental releases. In order to release to master every commit must be in the staging branch first. 

NOTE: build will be fixed by this PR https://github.com/socialcast/socialcast-git-extensions/pull/66
Instead of using 'staging' for the branch here, does it make sense to use last_known_good_staging instead to verify that the branch is both in staging and built successfully?