Update broccoli-caching-writer.

There was a bug in the `canLink` test functionality in 0.4.0 and 0.4.1
that caused an uncaught exception to be thrown in environments that did
not support linking (VMWare Fusion was the example provided).
Merge pull request #1963 from rwjblue/update-caching-writer

Update broccoli-caching-writer.