Update vitamin-r3 to 3.00b2; add stanzas
Update vitamin-r3 to 3.00b2; add stanzas

This commit makes the following changes:

- Bump version
- Add `auto_updates true`
- Add minimum macOS version (source: Sparkle metadata, homepage)

Note: If upstream keeps pushing releases so frequently without
versioned URLs, I’m going to remove the appcast and switch the
version to `:latest`.

lantrix/homebrew-versions,rogeriopradoj/homebrew-versions,mahori/homebrew-cask-versions,caskroom/homebrew-versions,victorpopkov/homebrew-versions,caskroom/homebrew-versions,Ngrd/homebrew-versions,Ngrd/homebrew-versions,pkq/homebrew-versions,404NetworkError/homebrew-versions,mahori/homebrew-versions,pkq/homebrew-versions,lantrix/homebrew-versions,rogeriopradoj/homebrew-versions,victorpopkov/homebrew-versions,404NetworkError/homebrew-versions