Add warning to script/server if curl does not support AsyncDNS

As discussed in #4202, add a warning to script/server if curl doesn't support Async DNS and thus Sidekiq might be unstable.

Should we close 4202 if we add this warning, I can also make sure the install instructions are maybe a little clearer? I guess since upstream is fixed already relating to 4202 and if we warn our users there is no reason to keep that bug open..