cleanup Gemfile, add development dependencies to gemspec.
More on why Gemfile.lock should not be included in version control system: http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/
Merge pull request #4 from tiramizoo/exclude-gemfile-lock

cleanup Gemfile, add development dependencies to gemspec.