[TASK] Configure Travis CI for better build performance

* set sudo: false to allow Travis to use the container-based infrastructure
* enable bundler caching
[TASK] Always use the latest Ruby 2.1 in Travis

Change "2.1.1" to "2.1" in .travis.yml to allow Travis to use the latest
2.1 point release. (Ruby uses Semantic Versioning starting with 2.1.0.)
Merge pull request #232 from braingourmets/task/ruby-versions

[TASK] Always use the latest Ruby 2.1 in Travis
Merge pull request #231 from braingourmets/task/travis-performance

[TASK] Configure Travis CI for better build performance