Update to latest version of `clean-css` to fix `nth` issues

[remcotolsma]

Two weeks ago there was a issue found with the output of:

.col-md-4:nth-child(3n+4) { clear: left; }

which was converted to:

.col-md-4:nth-child(3+4){clear:left;}

The 3n+4 part was converted to 3+4: clean-css/clean-css#899.

Are there plans to update clean-css in the grunt-contrib-cssmin library anytime soon?
https://github.com/gruntjs/grunt-contrib-cssmin/blob/v2.0.0/package.json#L20

[XhmikosR]

We already use the latest version. Make sure you clean up tyour node_modules dir and reinstall.