always remove host-only DHCP servers

A fresh install of VirtualBox automatically creates a host-only
interface (i.e., vboxnet0), with the DHCP server enabled.  This change
ensures that all DHCP servers are removed whether using Vagrant or
VirtualBox directly.

Fixes #39.
Merge pull request #92 from ericvw/dhcpserver-fix

always remove host-only DHCP servers