puppet: Only use those "modern" options when on xenial.

On trusty, we of course have an older version -- 1.4.14 -- and it is
not so modern, so this just gives an error.

puppet/zulip/templates/memcached.conf.template.erb

@@ -34,8 +34,10 @@ logfile /var/log/memcached.log
 # it's listening on a firewalled interface.
 -l 127.0.0.1
 
+<% if scope["zulip::base::release_name"] == "xenial" -%>
 # Use "modern" features.
 -o slab_reassign,slab_automove,lru_crawler,lru_maintainer,maxconns_fast,hash_algorithm=murmur3
+<% end -%>
 
 # Limit the number of simultaneous incoming connections. The daemon default is 1024
 # -c 1024