Fix 'useref' package reference & update dependencies

Due to the change introduced in digisfera/useref@a7a1e578b13e312dcfd824353bbf4a22d719e2e6 running `npm install` currently gives the following error message: 

```bash
npm ERR! notarget No compatible version found: useref@'>=0.2.6-0 <0.3.0-0'
npm ERR! notarget Valid install targets:
npm ERR! notarget ["0.0.1","0.1.0","0.2.0"]
```

Fixed the reference and updated `gulp` and `gulp-jshint` while I was at it. Thanks for this great plugin! :)