Update with_items loops to use new-ish "loop" keyword

Ansible 2.4 and 2.5 are moving away from specialized loop functions
and the old syntax will eventually be deprecated and removed. I did
not change the with_fileglob loops because I'm not sure about their
syntax yet.

See: https://docs.ansible.com/ansible/latest/user_guide/playbooks_loops.html

roles/nginx/tasks/letsencrypt.yml

@@ -18,7 +18,7 @@
 - name: Install certbot dependencies (Ubuntu 16.04)
   when: ansible_distribution == 'Ubuntu' and ansible_distribution_version is version_compare('16.04', '==')
   apt: name={{ item }} state=present update_cache=yes
-  with_items:
+  loop:
     - augeas-doc
     - augeas-tools
     - binutils
@@ -62,7 +62,7 @@
 - name: Install certbot dependencies (Debian 9)
   when: ansible_distribution == 'Debian' and ansible_distribution_major_version is version_compare('9', '==')
   apt: name={{ item }} state=present update_cache=yes
-  with_items:
+  loop:
     - augeas-doc
     - augeas-tools
     - autoconf