Use cssnano instead of clean-css => reduce minified CSS by 8.6% (4.9% gzipped)

Created by: pvdlg

	Minified size	Minified gain	Minified + zip size	Minified + zip gain
clean-css	150,996b	-	20,648b	-
cssnano	137,984b	8.6%	19,628b	4.9%