Update clean-css to v3.x.

/CC @kangax @duncanbeevers in case you sort this faster.

TODO

- [ ] Fix tests!!!
This regexp seems overly-complicated. Can it be replaced like so?

```javascript
var matches = text.match(/^\*\{([\s\S]*)\}$/m);
```

![regexps](https://cloud.githubusercontent.com/assets/7367/5541371/15e3a99a-8a9d-11e4-9e8f-ce733e26f688.png)

This uses fewer capture groups, and accepts all characters.