- Use the `HOMEBREW_PORTABLE_RUBY_VERSION` environment variable to
determine the version of the Portable Ruby to use in
`vendor-install.sh` and `ruby.sh`.
- Replace the `docs/.ruby-version` file with a symlink to
`Library/Homebrew/.ruby-version`.
- Fix an incorrect `HOMEBREW_LIBRARY` comment.
- Use a simpler `HOMEBREW_USING_PORTABLE_RUBY` definition.