unicorn 5.6.0 - early_hints support

This release adds support for the early_hints configurator
directive for the 'rack.early_hints' API used by Rails 5.2+.

Thanks to Jean Boussier for the patch.

Link: https://yhbt.net/unicorn-public/242F0859-0F83-4F14-A0FF-5BE392BB01E6@shopify.com/