Fix our WP Skeleton for Vagrant 1.3+

I just cloned our skeleton to start a new project and realised I haven't used it since Vagrant 1.3+ was released so I ran into an issue with with :extra as it's now referred to as :mount_options and takes an array.

@rmccue Can you please run your eyes over this and test it please. If you're happy with it then merge it in :smile: