Improve network performance for VirtualBox

Without those lines, VitrualBox machines are being very slow with all network operations (tested only in OS X Lion, but they are supposed to work everywhere). See https://github.com/mitchellh/vagrant/issues/1807

Vagrantfile

@@ -76,6 +76,8 @@ Vagrant::VERSION >= "1.1.0" and Vagrant.configure("2") do |config|
   config.vm.provider :virtualbox do |vb|
     config.vm.box = BOX_NAME
     config.vm.box_url = BOX_URI
+    vb.customize ["modifyvm", :id, "--natdnshostresolver1", "on"]
+    vb.customize ["modifyvm", :id, "--natdnsproxy1", "on"]
   end
 end