Bump webpack-dev-server from 2.11.3 to 3.1.11

Bumps [webpack-dev-server](https://github.com/webpack/webpack-dev-server) from 2.11.3 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/compare/v2.11.3...v3.1.11)

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

package.json

@@ -54,7 +54,7 @@
     "smart-banner-webpack-plugin": "^3.0.1",
     "style-loader": "^0.19.1",
     "webpack": "^3.6.0",
-    "webpack-dev-server": "^2.8.2"
+    "webpack-dev-server": "^3.1.11"
   },
   "scripts": {
     "prebuild": "rimraf static/js/webslide*",