Kick off a test travis build
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
Trigger Travis warm-cache build
Stop Travis dependency cache from being invalidated unnecessarily

Storing an updated build cache for this project takes around 2 minutes,
so we want to avoid any unnecessary cache-invalidation caused by spurious
file-changes.

Examples:

1. the timestamp being touched on the empty sbt .lock files:

    change detected: /home/travis/.sbt/boot/scala-2.10.4/org.scala-sbt/sbt/0.13.7/sbt.components.lock

2. version-ranged artifacts in Ivy (pure evil)

https://twitter.com/rtyley/status/545530331672371200

http://docs.travis-ci.com/user/build-configuration/#script
http://docs.travis-ci.com/user/languages/scala/#Default-Test-Command
Trigger another warm-cache build, now that we're not invalidating it
Merge pull request #42 from guardian/travis-docker-caching

Enable Travis dependency-caching with docker