fix(TaskProxy): Bluebird usage
fix(TaskProxy): Bluebird usage

`promisify` was using the wrong argument syntax.
Fixing this also fixes the `cache.clear()` method - previously it
returned `undefined` errors.

:LINK: http://bluebirdjs.com/docs/api/promise.promisify.html

(Pull Request URL: https://github.com/jgable/gulp-cache/pull/50)

jgable/gulp-cache