Merge pull request #310 from spotify/dxia/upgrade-docker-client

Upgrade docker-client dep to 2.7.3
unban the tag latest

Used to be because we would keep the old image and not re-pull,
but since we always attempt a pull, it is not nearly as evil
as it once was.  Still though, use only in test environments.
Merge pull request #313 from spotify/drewc/unban-latest

unban the tag latest
Upgrade docker-client dep to 2.7.4