Use Ansible's version_compare instead of doing math on strings

I'm surprised this worked all these years actually. Since Ansible
version 1.6 it has been possible to use the version_compare filter
instead of doing math logic on strings.

See: https://docs.ansible.com/ansible/playbooks_tests.html

roles/nginx/tasks/letsencrypt.yml

@@ -22,7 +22,7 @@
 # dependencies certbot checks for on its first run
 # taken from running certbot right after a clean Ubuntu 16.04 install
 - name: Install certbot dependencies (Ubuntu 16.04)
-  when: ansible_distribution == 'Ubuntu' and ansible_distribution_version == '16.04'
+  when: ansible_distribution == 'Ubuntu' and ansible_distribution_version | version_compare('16.04', '==')
   apt: name={{ item }} state=present update_cache=yes
   with_items:
     - augeas-doc