Sometimes you need to remove a package from your System, this is the way.
First discover the packages which are bothering you:
dpkg -l | grep ruby
Once you catched them, purge them from the system (removing binaries and configuration files):
sudo dpkg -P libruby1.8
Another collaboration with Tealeaf Academy was about how to use the brand new Capistrano 3 to deploy your applications; a basic guide:
I am collaborating with Tealeaf Academy writting in their blog, and my first post was about how to create a Development environment in Ubuntu: