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

Closes #57

Author: greenkeeper[bot]

package.json

@@ -45,7 +45,7 @@
     "eslint-config-prev": "^6.0.0",
     "extract-text-webpack-plugin": "^3.0.0",
     "gh-pages": "^1.0.0",
-    "html-webpack-plugin": "^3.0.0",
+    "html-webpack-plugin": "^4.2.1",
     "offline-plugin": "^4.8.3",
     "optimize-js-plugin": "^0.0.4",
     "react-hot-loader": "^4.0.1",