Download the latest Ubuntu (14.10 as of now) from the Ubuntu site and make a virtual machine using that ISO via virtualbox.
When you are done installing Ubuntu, run the following in a terminal to satisfy various dependancies. You can copy and paste this into the terminal fully or line by line.
sudo apt-get update && sudo apt-get upgrade -y
sudo apt-get install -y ruby-dev git g++
sudo gem update # This one takes a while and generates an error, it's ok.