fix clean-css usage

When the [clean-css dependency][1] was updated, it broke this plugin. Version 2.x introduces a [new syntax for usage][2]. This pull-request makes this plugin compatible with clean-css 2.x.

[1]:https://github.com/semfact/wintersmith-node-sass/commit/7bea7127f390d79741ca375b874fc77f74550476#diff-b9cfc7f2cdf78a7f4b91a753d10865a2R18
[2]:https://github.com/GoalSmashers/clean-css#how-to-upgrade-clean-css-from-1x-to-2x