Fix Rubinius build when libyaml is present in Homebrew

Rubinius never supported the `--with-libyaml-dir` configure option, so
we can never use `--if needs_yaml` since it's incompatible and might
break the build if libyaml was auto-discovered from Homebrew.

Starting from 2.2.0, Rubinius doesn't even need libyaml anymore since it
stopped building psych. Instead it requires users to install psych as a
gem which vendors its own libyaml anyway.