minor vulnerability in clean-css <4.1.11

[campbellgoe]

Please update clean-css to 4.1.11 or higher due to a minor vulnerability detailed here:
https://snyk.io/vuln/npm:clean-css:20180306

[scniro]

Thank you for bringing this to my attention. Will be fixed today and I'll post back here once it's published up.

[scniro]

@gcdeveloper fixed with the 3.9.3 release