Grunt 0.4.0rc5 changes the grunt.file.expand API

The commit gruntjs/grunt@b60907b7b07065a41325cd8a185d1a478b69f25a removed `grunt.file.expandFiles()` and `grunt.file.expandDirs()`. This PR addresses that change in `tasks/s3.js`.

These changes from grunt are not backwards compatible, and break the `s3` task completely.