Bump webpack-dev-server from 3.3.1 to 3.4.1 (#445)

Bumps [webpack-dev-server](https://github.com/webpack/webpack-dev-server) from 3.3.1 to 3.4.1.
- [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@v3.3.1...v3.4.1)

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