Permit script to run headless (do not require desktop session).

If no Linux desktop session is active, `VBoxManage startvm` can fail with an unclear error message ("Virtualbox.org #7367":https://www.virtualbox.org/ticket/7367

    Waiting for the VM to power on... 
    ERROR: The virtual machine 'debian-wheezy-64' has terminated unexpectedly during startup with exit code 0
    Details: code NS_ERROR_FAILURE (0x80004005), component Machine, interface IMachine, callee

Setting `--type headless` solved this issue for me (Ubuntu precise / 4.1.12_Ubuntur77245, Ubuntu saucy / 4.2.16_Ubuntur86992). I was not able to replicate this issue when remotely shelled into OSX.