add ability to skip broken imagemagick on shared hosting

When people started using png's, the poller started crashing on my system with this error:
> php-5.4: magick/splay-tree.c:880: GetValueFromSplayTree: Assertion `splay_tree != (SplayTreeInfo *) ((void *)0)' failed.
> /bin/sh: line 1: 31812 Aborted                 /usr/local/bin/php-5.4 include/poller.php

Running 'convert randomimg.png randomimg.jpg' on the same host causes the same error.
It appears to be this https://www.mail-archive.com/wikibugs-l@lists.wikimedia.org/msg92849.html
upstream: http://www.imagemagick.org/discourse-server/viewtopic.php?f=3&t=18136

People on shared hosting may wish to just disable imagemagick as a workaround.