Compatibility to gruntjs since 0.4.0-rc5

cowboy has removed <code>grunt.file.expandFiles</code> and <code>grunt.file.expandDirs</code>, see https://github.com/gruntjs/grunt/commit/b60907b7b07065a41325cd8a185d1a478b69f25a. I've just changed grunt-hub for you. ^^