Vagrantfile: Avoid DNS rebind protection by using TEST-NET-1

The [TEST-NET subnets](http://vektra.com/blog/test-net) are specified in RFC 5737 for use in documentation, and should be fine for our host-only networking uses.

dnsmasq (used in many home/prosumer/smb routers) rebind protection uses this list which notably excludes all three TEST-NETs:

- 127.0.0.0/8    (loopback)
- 192.168.0.0/16 (private)
- 10.0.0.0/8     (private)
- 172.16.0.0/12  (private)
- 169.254.0.0/16 (zeroconf)

I will update the DNS entries as soon as this is merged.