If a relative :path is in Berksfile, keep it relative in Berksfile.lock.

It's inconvenient to have absolute paths in Berksfile.lock 
when it is under version control and shared.
Merge pull request #310 from rectalogic/relative-path

If a relative :path is in Berksfile, keep it relative in Berksfile.lock.
style refactors from previous commit