Fix MSIE conditionals regex in minify output

Allows IE conditionals like the following to remain unmodified (from html5boilerplate):

```html
<!--[if gt IE 8]><!--> <html class="no-js"> <!--<![endif]-->
```
Without the change, the comment is not fully removed, remains unclosed, and the entire document is treated as commented!

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