Change how dependencies get loaded

When a project installed this gem, its dependencies would not be
available. I'm moving the dependencies to the Rakefile instead of the
Gemfile. Per Yehuda Katz
(http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/),
I'm also removing the Gemfile.lock from version control.