Update clean-css to ~4.0.3.

[XhmikosR]

DO NOT MERGE YET

@vladikoff: things to do before we merge this:

• [x] Update docs with any changed clean-css options
• [ ] Decide if it's going to be a major version bump; I vote yes.

[XhmikosR]

ping @vladikoff

[XhmikosR]

@jakubpawlowicz: things seem to work fine, but since you know better are there any API changes in v4?

We basically do https://github.com/gruntjs/grunt-contrib-cssmin/blob/master/tasks/cssmin.js#L44

Otherwise I think we can merge this and release a new version. :)

[XhmikosR]

Ping @vladikoff

[XhmikosR]

@vladikoff: please npm publish this; I don't have rights for this repo.

[XhmikosR]

@vladikoff ^^

[XhmikosR]

@vladikoff: ping

[vladikoff]

@XhmikosR Done! Sorry for the delay, I added you to npm for this package 👍

[XhmikosR]

Thanks!