_isVVVDir fails unnecessarily if no top-level vvv-hosts is present

[goldenapples]

When first trying to bootstrap a site with this script, I kept getting the That does not look like a VVV directory :( error message.

Checking to see why that would be, this check for a vvv-hosts file directly in the www directory turned out to be the culprit.

Because having this file isn't required in VVV (I like to keep my hosts definitions a level further down, in the site directories, so that if I remove a site, the hosts associated with it will also be removed), I think it would be good not to require that it be present.