Pass intnet as String

Otherwise we get:

```
/Applications/Vagrant/embedded/gems/gems/childprocess-0.3.9/lib/childprocess/abstract_process.rb:36:in `initialize': all arguments must be String: ["/usr/bin/VBoxManage", "modifyvm", "099abf87-2cc9-467a-bf7e-ef23ff620c90", "--nic1", "nat", "--nic2", "intnet", "--intnet2", true] (ArgumentError)
```