Ensure that both cache and dest dirs are cleaned up.

broccoli-writer has its own `cleanup` function, to clear the destination
directory up. The recent changes (to keep another cache directory) added
a `cleanup` function, but that overwrote the one we inherited from
`Writer`.
Merge pull request #21 from rwjblue/ensure-parent-is-cleaned-up-too

Ensure that both cache and dest dirs are cleaned up.