Configure Travis for better build performance

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

Change-Id: I5527d446e40688d2b564244c57883c4837dad77b
Reviewed-on: http://gerrit.causes.com/45436
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>