chore(package): update cssnano to version 4.0.5

Closes #222

Author: greenkeeper[bot]

package.json

@@ -107,7 +107,7 @@
     "cross-env": "^5.0.1",
     "cross-spawn": "^5.0.1",
     "css-loader": "^0.28.7",
-    "cssnano": "^3.10.0",
+    "cssnano": "^4.0.5",
     "exports-loader": "^0.6.3",
     "expose-loader": "^0.7.3",
     "file-loader": "^1.1.0",