Enable Travis dependency-caching with docker

Enable the new [docker-caching](http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/) feature for this SBT build. Build-time drops by 40%!

![image](https://cloud.githubusercontent.com/assets/52038/5487892/e61cb8ce-86ae-11e4-8593-8a51a8d46341.png)

Note that storing an updated build cache for this project takes around 2 minutes, so we want to avoid unnecessary cache-invalidations (sbt `.lock` files and lots of `.properties` files from Ivy dependencies that use [version-ranges](https://twitter.com/rtyley/status/545530331672371200)).