Issue #1909: Use rebuilt VM image for VirtualBox

This PR switches Vagrant over to a newly rebuilt VM image, for VirtualBox. I don't have VMWare, so I'm not sure how to rebuild there. (@jezdez?)

To try this out, you'll need to do something like this to destroy an existing VM and redownload the image:
```
vagrant destroy
vagrant box remove kuma-ubuntu
rm -rf kumascript/node_modules/fibers
vagrant up
```
The removal of the compiled fibers module under kumascript is a weird kink that probably needs a better solution: When the binary part of fibers gets compiled for the VM, it's stored in the project filesystem. It should really be stored somewhere in the VM itself, so that we don't end up with incompatible binaries after a VM update.