Update source location for Berkshelf 3.0

It seems changed from 3.0.0 (https://github.com/berkshelf/berkshelf/commit/90019cbfc23c3bd0c22d0eb1276226d71cf0eabe).
Hmm, it might be possible that this code gets executed (forced by config) even if Berkshelf gem is not loaded. So we should also check if `::Berkshelf::VERSION` is defined at all.