rack is optional at runtime, required for dev
We do not want to pull in a newer or older version of rack depending
on an the application running under it requires. Furthermore, it
has always been possible to use unicorn without any middleware at
all.
Without rack, we'll be missing descriptive status text in the first
response line, but any valid HTTP/1.x parser should be able to
handle it properly.
ref:
http://bogomips.org/unicorn-public/
20160121201255.GA6186@dcvr.yhbt.net/t/#u
Thanks-to: Adam Duke <adam.v.duke@gmail.com>
Thanks-to: Aaron Patterson <tenderlove@ruby-lang.org>