Instruct the builder that we don't need a cache dir.

[rwjblue]

Upcoming changes to broccoli itself will expose this as configuration. broccoli-funnel does not utilize the provided cache directory, so this just tells broccoli to avoid the extraneous work.

[joliss]

Looks good to me; feel free to merge whenever. (Remember to update the broccoli-plugin dependency as well once it's out.)