add file directory for vendor Ruby

The vendor Ruby will be put inside `Library/Homebrew/vendor/portable-ruby/<version>`,
with a symlink `Library/Homebrew/vendor/portable-ruby/current` pointed to it.

In addition, a `Library/Homebrew/vendor/portable-ruby-version` will
track the latest version of vendor binaries.

This gives us version control on vendor Ruby and enables us to bump vendor
Ruby whenever needed such as security update.