Bump webpack-dev-server from 2.11.5 to 3.1.11 in /test/dummy_sprockets

Bumps [webpack-dev-server](https://github.com/webpack/webpack-dev-server) from 2.11.5 to 3.1.11.
- [Release notes](https://github.com/webpack/webpack-dev-server/releases)
- [Changelog](https://github.com/webpack/webpack-dev-server/blob/master/CHANGELOG.md)
- [Commits](https://github.com/webpack/webpack-dev-server/commits/v3.1.11)

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

Author: dependabot[bot]

test/dummy_sprockets/package.json

@@ -32,6 +32,6 @@
     "webpack-merge": "^4.1.0"
   },
   "devDependencies": {
-    "webpack-dev-server": "^2.4.5"
+    "webpack-dev-server": "^3.1.11"
   }
 }