Use broccoli-filter's write() method

I think that all filter "sub-plugins" should use broccoli-filter's `write` method as it was intended to work, i.e. don't override it.

This makes `broccoli-jshint` work with symlinks as updated in https://github.com/broccolijs/broccoli-filter/pull/11 and also works with broccoli-filter master. And it's simpler this way :)