Update memory limit
Update memory limit

Based on composer/composer as reported by @reinink. Update memory
limit to 512MB if it is either less than 512MB or not -1 (no
memory limit).

Thanks Composer. :)

Closes #86

MalangPHP/sculpin,opengbu/sculpin,cyberid41/sculpin,beryllium/sculpin,sculpin/sculpin,sculpin/sculpin,davereid/sculpin,opdavies/sculpin,carbontwelve/sculpin,thomas-p-wilson/sculpin,lury/sculpin,vrkansagara/sculpin,bangpound/sculpin,steveleec/sculpin,simensen/sculpin,brianjking/sculpin,WouterJ/sculpin,beryllium/sculpin