Fix WinRM provisioning problem of issue #65

Here is my bugfix for issue #65. Adding the `WaitForCommunicator` in the `action_boot` function seems to do it for both unix guests (they still work) and Windows guests running the WinRM communicator of Vagrant 1.6.

The `WaitForCommunicator` exists in Vagrant 1.5.1 (haven't looked in older versions), so this should be backward compatible.