Restrict nginx resolver to ipv4

- Related issue #361

install/deb/nginx/nginx.conf

@@ -114,7 +114,7 @@ http {
     ssl_session_tickets off;
     ssl_stapling        on;
     ssl_stapling_verify on;
-    resolver 1.0.0.1 1.1.1.1 valid=300s;
+    resolver 1.0.0.1 1.1.1.1 valid=300s ipv6=off;
     resolver_timeout    5s;
     add_header          X-Frame-Options SAMEORIGIN;
     add_header          X-Content-Type-Options nosniff;