chore(package): update cssnano to version 4.1.9

Author: greenkeeper[bot]

package.json

@@ -11,7 +11,7 @@
   },
   "devDependencies": {
     "autoprefixer": "9.4.7",
-    "cssnano": "4.1.8",
+    "cssnano": "4.1.9",
     "dart-sass": "1.17.0",
     "fibers": "3.1.1",
     "grunt": "~1",