I just installed libvips through homebrew, following the instructions here and got version vips-7.40.11. Next i ran gem install ruby-vips for version 0.3.9, which fails with the following message:

extconf.rb:27:in `<main>': no pkg_config for any of following libvips versions: 7.29, 7.28, 7.27, 7.26, 7.24, 7.23, 7.22, 7.20 (RuntimeError)

It would seem that ruby-vips doesn't know how to build against v. 7.40?