Why is a dependency not being installed in a Ruby project?

Question

ruby gems

I'm trying to figure out a Ruby project. Never dealt with him.
The project uses logstash-devutils dependencies.
At the same time, the search for GEMs finds a dependency, but refuses to install it
. What could be the problem? And how to resolve it?
UPD.
ruby version
2.3.0p0

Answer 1

Understood. I had to install JRuby.