Fall back to latest virtualbox version if no box

If no box has been built for the specific version of virtualbox used,
print a warning but fall back to using the latest available box.

This code gets executed when you have vmware installed, and would fail
to run the Vagrantfile. I have also tried with later versions of
virtualbox, and the box has worked even though the version doesnt
exactly match, so I think printing a warning is better behaviour than
bailing completely.
Merge pull request #675 from alphagov/virtualbox-matching-version-fallback

Fall back to latest virtualbox version if no box