$ sudo apt-get update
$ sudo apt-get install plasma-scriptengine-ruby 

plasma-scriptengine-ruby is installed in your system.

Make ensure the plasma-scriptengine-ruby package were installed using the commands given below,

$ sudo dpkg-query -l | grep plasma-scriptengine-ruby *

You will get with plasma-scriptengine-ruby package name, version, architecture and description in a table

plasma-scriptengine-ruby package basic information: