Using ruby-install, homebrew building it for use with chruby, here’s how I install Rubinius under Yosemite (works for Mavericks as well.)

1. Make sure llvm is installed

    $ brew install llvm
   

2. Prepend the homebrew-installed llvm tools to your path

    $ export PATH="$(brew --prefix llvm)/bin:$PATH"
   
    # Or, for ZSH
    $ path=( $(brew --prefix llvm)/bin $path )
   

3. Install rubinius, v2.3.0 at the time of writing

    $ ruby-install rbx 2.3.0
   

4. Open a fresh shell once that’s built, and you should be able to switch to rbx!

    $ chruby rbx
    $ ruby -v
    rubinius 2.3.0 (2.1.0 9d61df5d 2014-10-31 3.5.0 JI) [x86_64-darwin14.0.0]
   

There is also a homebrew tap for rubinius which should also work instead of the above. I couldn’t get it working on one of my laptops though, which is why I was installing by hand using the above instead. The tap is at https://github.com/rubinius/homebrew-apps/ and https://twitter.com/brixen/status/529725881498226688 explains install steps.