Add a flag that can be used to clear the buildpack cache

In reference to issue #13.

`heroku config:set CLEAR_BUILDPACK_CACHE=` can be added to the heroku environment to clear the build cache.
Also requires `user-env-compile` to be enabled (via `heroku labs:enable user-env-compile`) in order to use the user environment in the buildpack.

* https://devcenter.heroku.com/articles/config-vars#setting-up-config-vars-for-a-deployed-application
* https://devcenter.heroku.com/articles/labs-user-env-compile