Bump webpack-dev-server from 2.11.5 to 3.1.12

Bumps [webpack-dev-server](https://github.com/webpack/webpack-dev-server) from 2.11.5 to 3.1.12.
- [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/v3.1.12)

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

package.json

@@ -48,7 +48,7 @@
     "style-loader": "^0.20.2",
     "wait-until-promise": "^1.0.0",
     "webpack": "3.12.0",
-    "webpack-dev-server": "^2.11.3",
+    "webpack-dev-server": "^3.1.12",
     "webpack-strip-block": "^0.2.0",
     "whatwg-fetch": "^3.0.0",
     "yamljs": "^0.2.8",