Fix MSIE conditionals regex in minify output func

Allows IE conditionals like the following to remain unmodified.

<!--[if gt IE 8]><!--> <html class="no-js"> <!--<![endif]-->

Credit to joebert regex from
http://www.sitepoint.com/forums/showthread.php?696559-Regex-pattern-to-strip-HTML-comments-but-leave-conditonals&s=3eef4ceb0a59b2fdb946fa56220fb6fd&p=4678083&viewfull=1#post4678083