Update Homebrew dependencies for Rubinius

The configure_ruby() function already assumes these dependencies are present:

https://github.com/postmodern/ruby-install/blob/1a78634bac21a7c90a0f01b7b618926662134238/share/ruby-install/rubinius/functions.sh#L26

so update this list to match.
Merge pull request #23 from jacknagel/patch-1

Update Homebrew dependencies for Rubinius
Rubinius vendors a fork of libffi.
Avoid running `pacman -Sy` for Arch users (closes #22).