Security: depends on webpack-dev-server < 3.1.6

[manuelkiessling]

Depending on webpack-dev-server < 3.1.6 at https://github.com/symfony/webpack-encore/blob/master/package.json#L54 introduces a "Missing Origin Validation" security issue of severity High, according to https://www.npmjs.com/advisories/725.