Add support for Ruby 2.1. Remove Gemfile.lock

I found that the library wasn't building on Ruby 2.1 despite the fact that it should support it no problem. I made the required changes to the Gemfile to do so.

Ruby best practices advices not to check in the [Gemfile.lock](http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/) into version control while developing a gem.