add port forwarding commands as vagrant triggers

When running on your host machine's local IP address it is
necessary to port forward 80 to 8080.  (When assigning a port to your
guest machine it must be larger than 1024, therefore we port forward
on the host machine so we can use the appropriate port).
Rather than setup and remove port forwarding
each time, I've added the commands as vagrant triggers.

The port forwarding rules are setup using pfctl

Note: this requires the Vagrant triggers add-on to be in place
https://github.com/emyl/vagrant-triggers

More information at
http://salferrarello.com/vagrant-vvv-setup-wordpress/
https://github.com/Varying-Vagrant-Vagrants/VVV/issues/442
https://github.com/Varying-Vagrant-Vagrants/VVV/pull/449#issuecomment-60941756