Bump rspec from 3.7.0 to 3.8.0 in /Library/Homebrew/test

Bumps [rspec](https://github.com/rspec/rspec) from 3.7.0 to 3.8.0.
- [Release notes](https://github.com/rspec/rspec/releases)
- [Commits](https://github.com/rspec/rspec/compare/v3.7.0...v3.8.0

)

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