The VM box "boxcutter/ubuntu1510" is not available for download. The "vagrant up" command for creating the development environment fails.
Changed to "boxcutter/ubuntu1604". Is this sound? It worked fine for me.
Use physical CPU count for virtual machine config.
VirtualBox recommends limiting CPUs to physical cores (hyperthreading cores don't count).
Tested on OS X 10.11.4 -- vagrant updated the VM on next boot.