Remove Gemfile.lock

At some point travis builds switched from using `bundle install` to
using `bundle install --deployment`, at which point the matrix builds
against multiple different puppet versions complain that the Gemfile
puppet version doesn't match Gemfile.lock any more.

If we're even bothering with matrix builds, it doesn't make sense to
have a Gemfile.lock, so let's just ditch it.
Merge pull request #5 from gds-operations/remove_gemfile_lock

Remove Gemfile.lock