chore(package): update cssnano to version 4.1.9

Closes #14

Author: greenkeeper[bot]

package.json

@@ -27,7 +27,7 @@
   "devDependencies": {
     "@savvy-css/spacing-variables": "^0.0.1",
     "@savvy-css/stylelint-config-savvy": "^0.2.0",
-    "cssnano": "^3.10.0",
+    "cssnano": "^4.1.9",
     "del-cli": "^1.0.0",
     "live-server": "^1.2.0",
     "nsp": "^2.6.3",