[CLEANUP] Change the PHPDoc type of $this->visitedNodes
[CLEANUP] Change the PHPDoc type of $this->visitedNodes

This field should be \DOMElement[], not \DOMNode[].

Fixes #186

Seldaek/emogrifier,OzzyCzech/emogrifier,MyIntervals/emogrifier,jjriv/emogrifier,sanderkruger/emogrifier,ricardofiorani/emogrifier,Synchro/emogrifier,ernsto/emogrifier