Add missing IE conditional comment.

It looks like the template is currently missing [one of the IE conditional comments](https://github.com/h5bp/html5-boilerplate/blob/6be0119cd2560e31a95383e5ba89963ca576dfea/index.html#L5), which I imagine results in multiple opening `html` tags in IE versions 8 and below.

This patch adds a helper in the style of [sporkd/compass-html5-boilerplate](https://github.com/sporkd/compass-html5-boilerplate/blob/d34fd479/lib/app/helpers/html5_boilerplate_helper.rb#L5).