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.