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!

Don't know how often pyro updates it's copy of CodeIgniter, but I have
also submitted this change to EllisLab/CodeIgniter#2120, with proper credit given there.