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

Closes #61

Author: greenkeeper[bot]

package.json

@@ -59,7 +59,7 @@
     "eslint-plugin-react": "^7.6.0",
     "extract-text-webpack-plugin": "^4.0.0-beta.0",
     "file-loader": "^1.1.6",
-    "html-webpack-plugin": "^3.1.0",
+    "html-webpack-plugin": "^4.2.1",
     "image-webpack-loader": "^4.0.0",
     "jest": "^22.3.0",
     "morgan": "^1.9.0",