Upgrade to gulp-util 2.2.0

This fixes the following on new versions of gulp:

```
.../node_modules/gulp/lib/createOutputStream/writeFile.js:7
  if (file.isNull()) return cb(null, file);
           ^
TypeError: Object #<File> has no method 'isNull'
    at module.exports (.../node_modules/gulp/lib/createOutputStream/writeFile.js:7:12)
    at .../node_modules/gulp/lib/createOutputStream/index.js:22:9
    at .../node_modules/gulp/node_modules/mkdirp/index.js:38:26
    at Object.oncomplete (fs.js:107:15)
```