Update Gemfile to use https rubygems, include Gemfile.lock

* Removes the warning about using the insecure http version of `rubygems.org`
* Checks in Gemfile.lock, [which is recommended to be included in projects unless they're gems themselves](http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/)