Tell developers where to download supported virtualbox editions

- reads like
```
Virtualbox version {YOURVERSION} is not supported by pp-development. See README.md.
Supported: ["SUPPORTED_VERSION"] --> ["http://download.virtualbox.org/virtualbox/SUPPORTED_VERSION"]
```

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.