Bump webpack-dev-server from 2.9.4 to 3.1.4

Bumps [webpack-dev-server](https://github.com/webpack/webpack-dev-server) from 2.9.4 to 3.1.4.
- [Release notes](https://github.com/webpack/webpack-dev-server/releases)
- [Commits](webpack/webpack-dev-server@v2.9.4...v3.1.4)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot-support