[Security] Bump debug from 2.6.8 to 2.6.9

Bumps [debug](https://github.com/visionmedia/debug) from 2.6.8 to 2.6.9. **This update includes a security fix.**
- [Release notes](https://github.com/visionmedia/debug/releases)
- [Changelog](https://github.com/visionmedia/debug/blob/2.6.9/CHANGELOG.md)
- [Commits](debug-js/debug@2.6.8...2.6.9)

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