Add support for CircleCI

CircleCI gives us faster, more reliable builds than Travis, since it has a
higher memory limit (4G vs 2G) and also has Docker running natively, rather
than us having to hack it into a usermode Linux instance.

* Add circle.yml configuration. It sets environment variables, ensures that
  Docker is exposed via TCP, and runs our Maven commands.

* Add a circle build profile.
Merge pull request #109 from spotify/rohan/circleci

Add support for CircleCI