WordPress dashboard plugin/theme installation/updates

Without this change, WordPress isn't able to install plugins/themes via the dashboard, or update plugins/themes/core via the WordPress dashboard.

This is a dirty hack to ensure that WordPress can install/update plugins, themes and core. It does this by making the /vagrant folder world writeable.

Note: for an already provisioned vagrant box, you'll probably have to run ```vagrant reload``` for these changes to take effect.

Keep up the great work @rmccue!