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

Closes #62

Author: greenkeeper[bot]

package.json

@@ -60,7 +60,7 @@
     "clean-webpack-plugin": "^3.0.0",
     "css-loader": "^3.2.0",
     "file-loader": "^4.2.0",
-    "html-webpack-plugin": "^3.2.0",
+    "html-webpack-plugin": "^4.2.1",
     "husky": "^3.0.9",
     "jest": "^24.9.0",
     "lint-staged": "^9.4.2",