Upgrade to Debian 8.0 Jessie

Roughly follow humgen/server-provisioning!1 (closed) taking the following into account:

After upgrading the system to Debian Jessie, but before running the Ansible playbook, remove the Mutalyzer Python virtual environment:

rm -Rf /opt/mutalyzer/virtualenv

Make sure all steps in the playbook get to run (e.g., cd /opt/mutalyzer/src/mutalyzer && git reset --hard HEAD^ force a repository change).