fix(cleancss): update to latest version of clean-css to mitigate issue with purging some css that should not be purged

Author: danbucholtz

package-lock.json

@@ -35,7 +35,7 @@
     "autoprefixer": "7.1.1",
     "chalk": "1.1.3",
     "chokidar": "1.7.0",
-    "clean-css": "3.4.27",
+    "clean-css": "4.1.9",
     "cross-spawn": "5.1.0",
     "express": "4.15.3",
     "fs-extra": "3.0.1",