Mininum virtualbox version

[jasonpartyka]

Hey @becw -- just wanted to let you know (and maybe it's already written down, apologies if it is), that I had to update my virtualbox version in order to use version 1.4. I was at 6.0.6 and ran into issues, because it seems that the guest additions in the 1.4 image are newer than that. I updated to 6.0.20 and things seem to be working ok.