chore(package): update cssnano to version 4.1.5

Author: greenkeeper[bot]

package.json

@@ -11,7 +11,7 @@
   },
   "devDependencies": {
     "autoprefixer": "9.2.1",
-    "cssnano": "4.1.4",
+    "cssnano": "4.1.5",
     "dart-sass": "1.14.2",
     "fibers": "3.0.0",
     "grunt": "~1",