Ignore Gemfile.lock

We had recently discussion about Gemfile.lock (!2507 (diffs, comment 595673161)), after giving it some thought I do not think we should keep this in source control. We should make sure GCS gem works with versions defined in gemspec instead of relying on Gemfile.lock.

More reasoning: https://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/

I'd love to get your opinion about that.