PHP Notices fixed.

The following error message appeared when using the script:

> Notice: Use of undefined constant urlencode - assumed 'urlencode' in /home/severin/dev/derpware/derptracker/trakt.php on line 17
> PHP Notice:  Use of undefined constant urlencode - assumed 'urlencode' in /home/severin/dev/derpware/derptracker/trakt.php on line 17
>
> Notice: Use of undefined constant slugify - assumed 'slugify' in /home/severin/dev/derpware/derptracker/trakt.php on line 17
> PHP Notice:  Use of undefined constant slugify - assumed 'slugify' in /home/severin/dev/derpware/derptracker/trakt.php on line 17

As it turned out, those were related to the dynamic function call setup used. Fixed that with this commit.