Tell developers where to download supported virtualbox editions

Aside: Whilst I dislike this approach that absolutely halts all Vagrant operations because of an idea of version support — including calls for non-write functions like the status of the current box — I think if we're doing this we should at least let people know where to find a copy of the supported version.
Set the link to the index of the version
Merge pull request #52 from alphagov/add_download_link

Tell developers where to download supported virtualbox editions