Blacklist rubinius and recommend ruby-build

Closes Homebrew/homebrew#23195.

Library/Homebrew/blacklist.rb

@@ -82,5 +82,11 @@ def blacklisted? name
     and then follow the tutorial:
       https://github.com/technomancy/leiningen/blob/stable/doc/TUTORIAL.md
     EOS
+  when 'rubinius' then <<-EOS.undent
+    Rubinius requires an existing Ruby 2.0 to bootstrap.
+    Doing this through Homebrew is error-prone. Instead, consider using
+    ruby-build` to build and install specific versions of Ruby:
+        brew install ruby-build
+    EOS
   end
 end