Use regular version of nginx on Arch

`nginx-mainline` requires modules be recompiled each time, and isn't handled automatically. It's still a very new and maintained release.

ansible/roles/gateway/files/nginx-cdn.conf

@@ -4,8 +4,7 @@ proxy_cache_path /var/lib/nginx/cache levels=1:2 keys_zone=cdncache:20m max_size
 
 {% for domain in cdn_domains %}
 server {
-    listen 8800 ssl proxy_protocol;
-    http2 on;
+    listen 8800 ssl http2 proxy_protocol;
 
     server_name {{ domain }};