Stop tracking Berksfile.lock

Causes all sorts of issue w/r/t shipping new versions, esp. when using
Stove. Process should look like so:

1. Finish work & cut new version using Stove
2. Stove bumps version number & ships updated metadata & CHANGELOG
3. Stove tags this new release
4. Run `bundle exec berks install` to update Berksfile.lock w/ this new
version number
5. Run `bundle exec berks upload` to push the newly cut version to the
Chef Server