Configure Travis for faster builds

* set sudo: false to allow Travis to use their new container-based
  infrastructure
* enable bundler caching

Change-Id: Ife4cb9c676f2e13876a05356039d35f01e00bcfb
Reviewed-on: http://gerrit.causes.com/45437
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>