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

Closes #353

Author: greenkeeper[bot]

package.json

@@ -64,7 +64,7 @@
     "eslint-plugin-react": "^7.0.0",
     "extract-text-webpack-plugin": "^3.0.0",
     "file-loader": "^1.1.7",
-    "html-webpack-plugin": "^3.0.0",
+    "html-webpack-plugin": "^4.2.1",
     "identity-obj-proxy": "^3.0.0",
     "jest": "^20.0.0",
     "json-loader": "^0.5.4",