Bump rack from 1.6.4 to 1.6.13
Bump rack from 1.6.4 to 1.6.13

Bumps [rack](https://github.com/rack/rack) from 1.6.4 to 1.6.13.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rack/rack/compare/1.6.4...1.6.13)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>
JulianNicholls/complete-photo-app,JulianNicholls/complete-photo-app,JulianNicholls/complete-photo-app