Update webpack-dev-server requirement to ^3.1.5 in /{{cookiecutter.app_name}}

Updates the requirements on [webpack-dev-server](https://github.com/webpack/webpack-dev-server) to permit the latest version.
- [Release notes](https://github.com/webpack/webpack-dev-server/releases)
- [Commits](https://github.com/webpack/webpack-dev-server/commits/v3.1.5)

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

{{cookiecutter.app_name}}/package.json

@@ -45,6 +45,6 @@
     "style-loader": "^0.18.2",
     "url-loader": "^0.5.9",
     "webpack": "^2.6.1",
-    "webpack-dev-server": "^2.4.5"
+    "webpack-dev-server": "^3.1.5"
   }
 }