Decouple the ruby recipe

The `supermarket::_ruby` recipe uses the `rubies` cookbook, which ends up
compiling from source the desired version of Ruby. This is highly undesirable
in our production supermarket environment as we do NOT want to be in the
business of unnecessarily compiling Ruby on machines. It adds several minutes
to the build time of new instances and it is obtuse. The `_ruby` previously
performed a package install from the "Brightbox" repositories. The
`supermarket::default` recipe still includes `supermarket::_ruby`, so a "full
stack" installation will continue to work (e.g., test kitchen, normal
standalone supermarket installs, etc).

While we're making progress on the omnibus supermarket, it's not done yet.
This change makes our staging and production environments not require a custom
compiled Ruby, as we can use our own `_ruby` recipe. Also, this ensures
maximum flexibility for others who may wish to install Ruby from local package
repositories, for example.