roles/nginx: Use stable release instead of mainline

roles/nginx/tasks/main.yml

 ---
 
 - name: install nginx
-  pacman: name=nginx-mainline,certbot state=present
+  pacman: name=nginx,certbot state=present
 
 - name: configure nginx
   template: src=nginx.conf.j2 dest=/etc/nginx/nginx.conf owner=root group=root mode=0644