Update Intro.php

The old version was not compatible with PHP5.4 and caused an White Screen of Dead when trying to access FTC-Intro in Contao-BE
Fix taken from:
http://stackoverflow.com/questions/8971261/php-5-4-call-time-pass-by-reference-easy-fix-available
Merge pull request #19 from stlorenz/master

Update Intro.php | now compatible with PHP5.4