bugfix: centos dns resolution issues because of natdns proxy.

Gusteau is currently forcing natdnsproxy on. This is causing dns 5
second dns resolution on
[Centos](https://github.com/mitchellh/vagrant/issues/1172). There is
more info on auto_nat_dns_proxy virtualbox setting
[here](https://github.com/mitchellh/vagrant/issues/1313).

e.g. The following has no effect:

  config.vm.provider :virtualbox do |vb|
    vb.auto_nat_dns_proxy = false
    vb.customize ["modifyvm", :id, "--natdnsproxy1",        "off"]
    vb.customize ["modifyvm", :id, "--natdnshostresolver1", "off"]
  end
Merge pull request #21 from jamesmoriarty/master

bugfix: centos dns resolution issues because of natdns proxy.