merge protections

1. Add extra condition to `mergable_by_kochiku?`: a build is not mergable if there is a newer build on the same branch
2. Set `build.merge_on_success` to false when aborting a build

Number 2 Is a necessary precaution because Kochiku will overturn an aborted build from aborted to success if all of its in-progress build attempts come back successful.

@square/kochiku-contributors