[BUGFIX] Remove Gemfile.lock.

Gemfile.lock must not be included in a gem as it needlessly narrows the
allowed versions of dependent gems, making the gem very hard to get to
work together with other gems.

Source:
http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/

Also add Gemfile.lock to .gitignore.