Force host dns resolution for guest

Issue on osx 10.9.2 with virtualbox 4.3.8 where dns queries through certain programs like curl were timing out. Error was thrown in vagrant up for bootstrap.saltstack.org . 

Adding in config line from :https://leap.se/code/issues/1089 resolved the issue.
Why'd you change the default here? The original justification for `default` is that the request really shouldn't be served if there's no explicit intention to. Also, `wordpress-trunk.dev` won't exist on production machines.