Use default synced folder at /vagrant

Fixes https://github.com/protobox/protobox-web/issues/14

I did notice that the config produced by protobox-web has ```nfs:false``` but ```type``` isn't specified so protobox defaults to nfs https://github.com/protobox/protobox/blob/master/Vagrantfile#L126

Let me know if this is intentional (will fix in protobox-web) or not (will fix in protobox)