Remove Gemfile.lock from gem

Gems should not have Gemfile.lock in version control, see
http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/

[ci skip]
Ignore Gemfile.lock in git

Gems should not have Gemfile.lock in version control, see
http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/

[ci skip]