We've shifted the default OS version in KohaDevBox to Debian Stretch. We will try to keep following stable. You can still run Debian Jessie by explicitly asking for it:
$ vagrant up jessie
$ vagrant ssh jessie
and so on.
If you already have a Jessie-based box running and update the kohadevbox code, please remember to add the trailing 'jessie' word to the commands you usually ask vagrant to run.