ruby.sh: allow running on older Ruby versions

[EricFromCanada]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
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• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

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When installing on a fresh pre-10.15 system, the installer throws an error because /usr/bin/ruby is 2.3.7, but Version.canonical_segments didn't arrive until 2.5.0.

...
 * [new tag]             2.2.7      -> 2.2.7
 * [new tag]             2.2.8      -> 2.2.8
 * [new tag]             2.2.9      -> 2.2.9
HEAD is now at 8e984d621 Merge pull request #7419 from Bo98/audit
-e:1:in `<main>': undefined method `canonical_segments' for #<Gem::Version "2.3.7"> (NoMethodError)
==> Downloading https://homebrew.bintray.com/bottles-portable-ruby/portable-ruby-2.6.3.mavericks.bottle.tar.gz
...

[Bo98]

Can you rebase on master to get the workflow changes?