You never use tools like rbenv or rvm on production servers. They exist so that mac devs can be running the same versions of ruby found in the Linux distros.
Godspeed Leader Nussbaum! I hope that your first action is to find a replacement for whoever was responsible for the debian-ruby disaster.
It is nice to rely on system-packages, but if you depend on certain versions of certain packages you might see problems with that approach.