Update clean-css to v2.0.x

Update clean-css to v2.0.x.
Changed api call from
```
var minimized = CleanCSS.process(source, options);
```
to
```
var minimized = new CleanCSS(options).minify(source);
```

As page of CleanCSS [says](https://github.com/GoalSmashers/clean-css#how-to-upgrade-clean-css-from-1x-to-2x "How to upgrade clean-css from 1.x to 2.x?")