Update webpack requirement in /packages/generator/app/templates

Updates the requirements on [webpack](https://github.com/webpack/webpack) to permit the latest version.
- [Release notes](https://github.com/webpack/webpack/releases)
- [Commits](webpack/webpack@v4.38.0...v4.39.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

packages/generator/app/templates/package.json

@@ -41,7 +41,7 @@
     "react": "^16.8.6",
     "react-dom": "^16.8.6",
     "redbox-react": "^1.6.0",
-    "webpack": "^4.38.0",
+    "webpack": "^4.39.0",
     "webpack-cli": "^3.3.6",
     "webpack-dev-server": "^3.7.2"
   },