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).
Remove extra nesting of `addon/styles` tree.

The prior code was actually looking for `addon/styles/styles/` which was
incorrect.
Merge pull request #1963 from rwjblue/update-caching-writer

Update broccoli-caching-writer.
Merge pull request #1964 from rwjblue/fix-nesting-of-addon-styles

Remove extra nesting of `addon/styles` tree.