Bump grunt-contrib-cssmin to ~0.11.0

After checking the resulting minified CSS, the only differences seem to be:
* Selectors get sorted; e.g. `.d, .c, .b {...} .a {...}` => `.b, .c, .d {...} .a {...}` Seems harmless. Have filed https://github.com/jakubpawlowicz/clean-css/issues/425 about using a smarter ordering though.
* `background: transparent !important;` now minifies to `background: 0 0 !important;` I'm not sure whether this matters

CC: @mdo @XhmikosR