Enable dependency-caching between builds in Travis

Here we're caching the .sbt & .ivy2 directories, both used for an SBT
build. This is based off the new docker-caching feature.

http://docs.travis-ci.com/user/workers/container-based-infrastructure/#Routing-your-build-to-container-based-infrastructure
http://docs.travis-ci.com/user/caching/#Arbitrary-directories