Fixes legacy port forwarding in Vagrantfile.pkg

```
[vagrant] Cleaning up downloaded box...
/Applications/Vagrant/embedded/gems/gems/vagrant-1.0.6/lib/vagrant/config/vm.rb:44:in `to_s': wrong number of arguments(1 for 0) (ArgumentError)
	from /Applications/Vagrant/embedded/gems/gems/vagrant-1.0.6/lib/vagrant/config/vm.rb:44:in `forward_port'
```

FIX: https://github.com/rtfd/readthedocs.org/pull/177