Get rid of AUTO_ROLLBACK, folding it in to AUTO_DEPLOY instead.

Summary:
There was a never a case that people would want to manually deploy but
automatically roll back.  So now AUTO_DEPLOY just does both:
automatically deploy, and (if necessary) automatically roll back.

Test Plan: Fingers crossed.

Reviewers: chris

Reviewed By: chris

Differential Revision: http://phabricator.khanacademy.org/D12822