fix(package): update html-webpack-plugin to version 4.0.4

Closes #257

Author: greenkeeper[bot]

package.json

@@ -51,7 +51,7 @@
     "extract-text-webpack-plugin": "^4.0.0-beta.0",
     "file-loader": "^1.1.6",
     "history": "^4.7.2",
-    "html-webpack-plugin": "^3.0.0",
+    "html-webpack-plugin": "^4.0.4",
     "jsonwebtoken": "^8.1.0",
     "material-ui": "^1.0.0-beta.29",
     "material-ui-icons": "^1.0.0-beta.17",