Closes #2988 "Update clean-css dependency in pug-filters"

[callmeaponte]

This is my first pull request (on a forked public repo at least), so please let me know if I'm missing anything.

None of the breaking changes in clean-css v4 should affect you, assuming this line in run-filter.js is the only place where you use it:

result = new CleanCSS().minify(result).styles;

This will close #2988.

[ForbesLindesay]

Thanks, all tests passed so I've released as 2.0.3. Congrats on your first pull request :)