Remove the Gemfile.lock from git index

http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/

I want to add yard as a development dependency to the gemspec.  The gemspec included all files under version control, which includes the Gemfile.lock.  If yard is part of the gemspec and the Gemfile.lock is part of the gem, the user will have to install yard to use calabash-android.