chore(package): update cssnano to version 4.1.2

Closes #12

Author: greenkeeper[bot]

package.json

@@ -31,7 +31,7 @@
     "ticketfly-css-spacing-variables": "^0.0.5"
   },
   "devDependencies": {
-    "cssnano": "^3.9.1",
+    "cssnano": "^4.1.2",
     "del-cli": "^0.2.1",
     "http-server": "^0.9.0",
     "npmpub": "^3.1.0",