chore(package): update html-webpack-plugin to version 4.0.3

Closes #107

Author: greenkeeper[bot]

package.json

@@ -67,7 +67,7 @@
     "eslint-plugin-react": "^7.16.0",
     "gh-pages": "^2.1.1",
     "highlight.js": "^9.16.2",
-    "html-webpack-plugin": "^3.2.0",
+    "html-webpack-plugin": "^4.0.3",
     "ignore-emit-webpack-plugin": "^2.0.2",
     "jest": "^24.9.0",
     "mini-css-extract-plugin": "^0.8.0",