[Security] Bump npm from 6.11.3 to 6.13.6

Bumps [npm](https://github.com/npm/cli) from 6.11.3 to 6.13.6. **This update includes security fixes.**
- [Release notes](https://github.com/npm/cli/releases)
- [Changelog](https://github.com/npm/cli/blob/latest/CHANGELOG.md)
- [Commits](npm/cli@v6.11.3...v6.13.6)

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