Added callback to let grunt know when we're done with async operations

I'm running grunt 0.4.5, with grunt-zopfli-native 0.4.1.
When I ran the task, I saw the output file being created, with size 0.

I guessed this was due to the asynchronous nature of the stream operations, so I modified the code according to [this page](http://gruntjs.com/api/inside-tasks).

As a side note, I also noticed that `grunt.util.async` is now deprecated, though still working at the moment. You may want to look into this at a later time.