Ah - silly me! I downloaded 2.2.23, but then inadvertently re-installed 2.2.7 (which I still had in my downloads directory from before)! I've installed 2.2.23 correctly now, and I do get the Ruby option on the basis tab. I've checked it for a site, but my test .rb script still doesn't work.
Do I need to do anything else? After checking the Ruby checkbox, and restarting Apache, I don't see anything about Ruby in the vhosts file here:
Should there be something there?