Install Namazu-Mailman integration package

ansible/roles/mailman/tasks/main.yml

-- name: Install Mailman 2 with Apache
+- name: Install Mailman 2 with Apache, Exim and nmzproc
   become: yes
   apt:
     name:
     - mailman
     - apache2
     - exim4-daemon-heavy # for the content scanning extension
+    - nmzproc
   register: install_mailman
 # mailman.service fails and tries to restart indefinitely (Site list is missing: mailman)