chore(package): update cssnano to version 4.0.4

Closes #30

Author: greenkeeper[bot]

package.json

@@ -51,7 +51,7 @@
     "babel-preset-stage-0": "^6.24.1",
     "babel-register": "^6.26.0",
     "css-loader": "^0.28.7",
-    "cssnano": "^3.10.0",
+    "cssnano": "^4.0.4",
     "eslint": "^4.16.0",
     "eslint-config-airbnb": "^16.1.0",
     "eslint-plugin-import": "^2.8.0",