Add ruby-version datasource

[rarkins]

This is a datasource to be used specifically for retrieving Ruby versions and therefore is not the same as Rubygems. This one will be used for updating .ruby-version files and ruby lines inside Gemfiles.

We need to include both stable and unstable releases.

Possible sources:

• https://www.ruby-lang.org/en/downloads/releases/ (includes date and link to release notes, which is handy)
• https://github.com/postmodern/ruby-versions/blob/master/ruby/versions.txt (simple and semantic. Hand-crafted so would lag the official releases by hours or days, however if we use ruby-install to build our own images then we probably need to wait for this to be updated anyway)
• https://github.com/ruby/ruby/releases (needs to be massaged into semantic versions)

I suspect that the first source is the best for us.

[rarkins]

Requirements:

• Query the ruby-lang.org downloads/releases page and parse the HTML
• Return homepage=https://ruby-lang.org and sourceUrl=https://github.com/ruby/ruby
• Return an array of releases, each with version, releaseDate and changelogUrl
• If the website fails or we can't parse it then we should throw registry-failure message to abort renovation
• Cache results for 15 minutes

[renovate-bot]

🎉 This issue has been resolved in version 13.178.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀