Bump webpack-dev-server from 2.11.1 to 5.0.1 in /client

Bumps [webpack-dev-server](https://github.com/webpack/webpack-dev-server) from 2.11.1 to 5.0.1.
- [Release notes](https://github.com/webpack/webpack-dev-server/releases)
- [Changelog](https://github.com/webpack/webpack-dev-server/blob/master/CHANGELOG.md)
- [Commits](webpack/webpack-dev-server@v2.11.1...v5.0.1)

---
updated-dependencies:
- dependency-name: webpack-dev-server
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

client/package.json

@@ -11,6 +11,6 @@
     "babel-preset-es2015": "^6.24.1",
     "babel-preset-react": "^6.24.1",
     "webpack": "3.11.0",
-    "webpack-dev-server": "2.11.1"
+    "webpack-dev-server": "5.0.1"
   }
 }