[Security] Bump slug from 0.9.1 to 0.9.4
[Security] Bump slug from 0.9.1 to 0.9.4

Bumps [slug](https://github.com/Trott/slug) from 0.9.1 to 0.9.4. **This update includes a security fix.**
- [Release notes](https://github.com/Trott/slug/releases)
- [Changelog](https://github.com/Trott/slug/blob/master/CHANGELOG)
- [Commits](https://github.com/Trott/slug/compare/0.9.1...v0.9.4)

Signed-off-by: dependabot-preview[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
mkungla/aframe-webpack-boilerplate,mkungla/aframe-webpack-boilerplate