Nothing on the system really uses Ruby should it should be safe to upgrade. I would download the slackbuild from
Download the version of Ruby you want and run the Slackbuild script. You probably want to comment out the --enable-pthread, as I don't think it the configure script for recent Ruby releases use that switch anymore.
Then you can install the resulting package with upgradepkg
I use Ruby quite a bit but have never compiled it form source on Slackware 1.9.3 has been good enough for. I have had to build from source to upgrade some RHEL systems off of the ancient 1.8.7 release they are still on. It does not have an excessive amount of dependencies or complex install requirements so I don't think you will have any problems.