Fixes #23: Remove Gemfile.lock from repo per Yehuda Katz

Move dev dependencies from Gemfile to Gemspec

Read these two articles:

1. [Clarifying the Roles of the .gemspec and Gemfile](http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/)
2. [Gem Development Best Practices](http://blog.carbonfive.com/2011/01/22/gem-development-best-practices/)