Fix problems transitioning from insecure_private_key to less_insecure_private_key (Vagrant >= 1.4.0)

This doesn't fix Vagrant < 1.4.0.

Without this, `vagrant reload` after installing the plugin just hangs
forever waiting for the VM to boot, with output similar to the
following:

    ==> default: Less insecure SSH key not found, generating key
    ==> default: Less insecure SSH key generated and stored at /Users/dlitz/.vagrant.d/less_insecure_private_key
    ==> default: Attempting graceful shutdown of VM...
        default: Guest communication could not be established! This is usually because
        default: SSH is not running, the authentication information was changed,
        default: or some other networking issue. Vagrant will force halt, if
        default: capable.
    ==> default: Stopping the VMware VM...
    ==> default: Verifying vmnet devices are healthy...
    ==> default: Preparing network adapters...
    ==> default: Starting the VMware VM...
    ==> default: Waiting for the VM to finish booting...
Merge pull request #4 from dlitz/key-transition-fix

Fix problems transitioning from insecure_private_key to less_insecure_private_key (Vagrant >= 1.4.0)