Update Vagrant base box for VMWare to fix error 404 on vagrant up

Tried to follow the suggested [Development Environment setup](https://github.com/al3x/sovereign/wiki/Development-Environment) but got a 404 error (vagrant and vmware fusion on osx).
- The url in vagrantfile for the vmware_fusion compatible box has error 404.
- The commented source of that box leads to deprecated project, the repo's contents having been moved to https://github.com/box-cutter/debian-vm
- Vagrant boxes built with that new repo are now available on Vagrant Cloud, I used the one for 'Debian Wheezy 7.5 (64-bit)' https://vagrantcloud.com/box-cutter/debian75

For me Vagrant provision is now failing with a [github hostkey](https://github.com/al3x/sovereign/issues/261) issue but that seems entirely separate from the missing box error – so I haven't yet been able to run the sovereign tests...