Faster Travis Builds with Caching

See http://about.travis-ci.org/docs/user/caching/

Use travis gem CLI to clear https://github.com/travis-ci/travis#cache

API ref https://api.travis-ci.com/docs/#/repos/:owner_name/:name/caches


h/t @moonglum https://github.com/hcking/hcking/blob/dd0c778a64c6478cdad89d14cda033551759312f/.travis.yml