Support Berkshelf 3.0

Berkshelf 3.0 will replace `Berksfile#install(:path => path)` with
`Berksfile#vendor(path)`. As we don't have hard dependency on it, we
need to support both.
Test with Berkshelf v3.0.0(.beta)
Merge pull request #268 from tmatilai/berkshelf-3.0

Support Berkshelf 3.0