Alex, I'm not sure I understand what you mean. No, I don't have Ruby 2.3.4 for build. If it was so, I expect it to build successfully. My Ruby version for build is coming from RMS, so 1.9.3 whereas the VM is 2.3.4. These 2 don't match. So solution is to upgrade the Ruby that comes with installer. Is that possible? If yes, when can we expect updated installer?
To reproduce, simply run installer in a Windows machine without any Ruby installed, create a simple extension, and compile. That error will show up.
I'm away from dev PC so cannot upload my source code, but I'm sure you can reproduce at your end with a clean install using installer.