remove Berksfile and Berksfile.lock from generated chefignore file

We recommend stripping the Berksfile or Berksfile.lock from cookbooks uploaded to the Chef Server. These files need to be redistributed to cookbook authors and available for tools who consume them.

Currently cookbooks retrieved via a Chef Server endpoint of a Berkshelf-API server will not contain the expected Berksfile and, if an Environment Cookbook, the Berksfile.lock.