Blacklist MacRuby

For now we don't support this kind of thing because our formula is a worse option, even though you have to update it yourself.

Closes Homebrew/homebrew#6174

Library/Homebrew/blacklist.rb

@@ -42,6 +42,12 @@ def blacklisted? name
 
         easy_install pip
     EOS
+  when 'macruby' then <<-EOS.undent
+    MacRuby works better when you install their package:
+      http://www.macruby.org/downloads.html
+
+    Although if you prefer, there is a formula in homebrew-alt.
+    EOS
   when 'npm' then <<-EOS.undent
     npm can be installed thusly by following the instructions at
       http://npmjs.org/