Provide recommended Vagrant and VirtualBox versions

[geerlingguy]

One thing to note—if you have a different VirtualBox version installed than the one my current box versions are configured for, it might cause some extra time per build installing updated Guest Additions (if you have vagrant-vbguest installed).

So I'd like to add a recommendation for what versions were used at the time the version was released.

In my case, I'm currently on VirtualBox 5.1.10, but 5.1.12 is the latest release...