Use with_first_found instead of the deprecated first_available_file

I was getting and error when running this role:
```
fatal: [localhost] => One or more undefined variables: 'item' is undefined
```

Updated documentation: http://docs.ansible.com/playbooks_loops.html#finding-first-matched-files

According to @mpdehaan `first_available_file` has been deprecated and `with_first_found` is the updated way.