chore(package): update optimize-css-assets-webpack-plugin to version 4.0.2

Closes #16

Author: greenkeeper[bot]

package.json

@@ -64,7 +64,7 @@
     "jest": "^22.3.0",
     "morgan": "^1.9.0",
     "nodemon": "^1.14.1",
-    "optimize-css-assets-webpack-plugin": "^3.2.0",
+    "optimize-css-assets-webpack-plugin": "^4.0.2",
     "react-test-renderer": "^16.2.0",
     "sass-loader": "^6.0.6",
     "style-loader": "^0.20.0",