Upgrade docker-client dep to 2.7.3
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
CircleCI: Ensure correct level of parallelism

This setting got flipped in CircleCI somehow and caused build failures.

Add a check for it at the very beginning of the build and print an
informative message in case it happens again.
Upgrade docker-client dep to 2.7.4
Merge pull request #315 from spotify/rohan/circleci

CircleCI: Ensure correct level of parallelism
Merge pull request #316 from spotify/dxia/upgrade-docker-client

Upgrade docker-client dep to 2.7.4
Rename --force to --yes in CLI

* Rename it because we use --force to mean
  skip the interactive prompt and answer yes
* Keep the old '--force' for now and show a warning

Fixes #311