buildRubyGem: fix missing libobjc in darwin

This was preventing any ruby gem with a c extension to build.

mkmf would fail with a misleading error:

    /nix/store/dmkcai8fnv21qxiasx628nim3mq4r4wg-ruby-2.2.3-p0/lib/ruby/2.2.0/mkmf.rb:456:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
    You have to install development tools first.