ruby: make keg-only (as provided by macOS).

[MikeMcQuaid]

This makes it consistent with the other Ruby formulae and avoids weird PATH issues (exactly what keg-only is designed for).

Discussion in Homebrew/brew#5155.

• Have you followed the guidelines for contributing?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

---