Upgrade to Ruby 2.1.1

I know Ruby was just bumped in #13 but 2.1.1 is actually the latest patch version and [has some nice performance improvements](https://www.ruby-lang.org/en/news/2014/02/24/ruby-2-1-1-is-released/)...

Better yet, while I'm usually a proponent of specifying a specific ruby, does it maybe make sense to not specify in the Gemfile and instead test against Travis or something so you know what versions the project can support?