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

Closes #60

Author: greenkeeper[bot]

packages/core/package.json

@@ -8,7 +8,7 @@
     "copy-webpack-plugin": "^4.5.2",
     "css-loader": "^1.0.0",
     "file-loader": "^3.0.0",
-    "html-webpack-plugin": "^3.2.0",
+    "html-webpack-plugin": "^4.2.1",
     "lodash.merge": "^4.6.1",
     "mini-css-extract-plugin": "^0.4.3",
     "node-sass": "^4.9.3",