Merge pull request #28 from less/dependabot/npm_and_yarn/clean-css-4.2.1

Bump clean-css from 3.4.28 to 4.2.1

Author: matthew-dean

package.json

@@ -20,7 +20,7 @@
     "node": ">=0.10"
   },
   "dependencies": {
-    "clean-css": "^3.0.1"
+    "clean-css": "^4.2.1"
   },
   "keywords": [
     "less plugins",