chore(package): update html-webpack-plugin to version 4.2.1

Closes #34

Author: greenkeeper[bot]

package.json

@@ -62,7 +62,7 @@
     "eslint-plugin-react": "^7.10.0",
     "file-loader": "^1.1.11",
     "flow-bin": "^0.77.0",
-    "html-webpack-plugin": "^3.2.0",
+    "html-webpack-plugin": "^4.2.1",
     "husky": "^1.0.0-rc.13",
     "json-loader": "^0.5.7",
     "mini-css-extract-plugin": "^0.4.1",