Updated W3 Total Cache Hash extract module

This updates the W3 Total Cache Hash Extract module to use the wordpress mixin and applies rubocop suggestions.

The vulnerable version was patched in the tag so to get the vulnerable version you have to execute the following statements in your `wp-content/plugins/` folder:
```
svn co -r 428882 http://plugins.svn.wordpress.org/w3-total-cache/tags/0.9.2.4/ w3-total-cache/
chown -R www-data:www-data w3-total-cache/
```

Output:
![11](https://cloud.githubusercontent.com/assets/105281/3739225/01820428-1749-11e4-8413-f25002bc80b7.png)