silence the selfoss cron job

roles/news/tasks/selfoss.yml

   notify: restart apache
 
 - name: Install selfoss cronjob
-  cron: name="selfoss" user="www-data" minute="*/5" job="curl -k 'https://{{ selfoss_domain }}/update' > /dev/null"
+  cron: name="selfoss" user="www-data" minute="*/5" job="curl --silent --show-error -k 'https://{{ selfoss_domain }}/update' > /dev/null"