Update uncss version

[Ryan Zimmerman]

I'm one of the maintainers of uncss.

uncss is now on version 0.15.0. We've switched to jsdom for HTML & JS parsing, so we now don't require any native dependencies (like PhantomJS). Is there any possibility this plugin could be updated to use the latest version of uncss, since we reference it from our readme?

CC: @mikelambert @XhmikosR

[Ryan Zimmerman]

Thanks!