Bump rack-cache from 1.7.2 to 1.8.0
Bump rack-cache from 1.7.2 to 1.8.0

Bumps [rack-cache](https://github.com/rtomayko/rack-cache) from 1.7.2 to 1.8.0.
- [Release notes](https://github.com/rtomayko/rack-cache/releases)
- [Changelog](https://github.com/rtomayko/rack-cache/blob/master/CHANGES)
- [Commits](https://github.com/rtomayko/rack-cache/compare/v1.7.2...v1.8.0)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
sul-dlss/rdf-resource,sul-dlss/rdf-resource