Hook WaitForCommunicator instead of Boot

[nrwiersma]

If you are hooking Boot, this will fail on first boot trying to send the plugin with communicator. Instead hook WaitForCommunicator, then we know we send the plugin.

[OndraM]

+1, hooking to the Boot event causes this (Vagrant 1.7.2) during vagrant up.

...
==> default: Forwarding ports...
    default: 22 => 2222 (adapter 1)
==> default: Running 'pre-boot' VM customizations...
==> default: Booting VM...
Vagrant timed out while attempting to connect via SSH. This usually
means that the VM booted, but there are issues with the SSH configuration                                                                                        
or network connectivity issues. Please try to `vagrant reload` or                                                                                                
`vagrant up` again

And with this fix it is working properly.

[fgrehm]

Sorry for the long silence but as per GH-24 I'm looking for help to maintain this plugin and for now I'm holding back from bringing in new changes =/

If you are interested on maintaining the project and keep things moving please shoot me an email or drop a message on GH-24 so I can arrange things.

Cheers 🍻

[alpha01]

Update has been applied to vagrant-notify 0.5.0.