Update broccoli-caching-writer dependents to allow linking fallback.

* Update broccoli-caching-writer to handle fallbacks if linking is not
  possible (for example in a vagrant box).
* Update broccoli-file-mover
* Update broccoli-file-remover
Merge pull request #1799 from rwjblue/update-to-enable-vagrant

Update broccoli-caching-writer dependents to allow linking fallback.