mathiasbynens/grunt-zopfli — pull request #15 by shinnn (2014-02-01 08:16:04+00:00)
Title: Remove grunt.util.async

[`grunt.util.async`](http://gruntjs.com/api/grunt.util#grunt.util.async) is [deprecated](http://gruntjs.com/blog/2013-11-21-grunt-0.4.2-released). So I replaced it with [async](https://github.com/caolan/async) module.