Earlier quoted context omitted.
Assuming he had a green light on the project itself, and it creates value, I don't see why that would be selfish.
Suppose the OP leaves. It's going to be hard for the his former coworkers to maintain the Ruby app.
Do what gets the job done, within reasonable limits. If the expected maintenance is minimal and you can improve your personal capability and/or produce the deliverable faster using a different technology, by all means go for it. I've had to learn languages far more unpleasant than Ruby (VBA, I'm looking right at you!!) in order to maintain programs that no one expected would ever change. That's life!