closes #43, closes #42: Fix Travis CI gradle cache
closes #43, closes #42: Fix Travis CI gradle cache

* Adds before_cache and cache explained in travis-ci's docs.

https://docs.travis-ci.com/user/languages/java/#Caching

healthbam/healthbam,healthbam/healthbam,RyanBard/healthbam,RyanBard/healthbam,healthbam/healthbam,RyanBard/healthbam