Update Virtualbox download note to 4.2.16, add guinea pig warning.

4.2.14 broke Vagrant in a very annoying way, forcing anyone that had upgrade or installed the latest to roll back to 4.2.12. Thankfully, 4.2.16 was just released and resolves this issue.

Added a referencing the older downloads page just in case a new release comes out and somebody finds these instructions before we have properly tested things.

Closes #90