unicorn 5.5.0

unicorn 5.5.0

Mostly identical to 5.5.0.pre1, which I didn't hear any feedback
from:

  https://bogomips.org/unicorn-public/20181220222842.GA27382@dcvr/

> Jeremy Evans contributed the "default_middleware" configuration option:
>
>   https://bogomips.org/unicorn-public/20180913192055.GD48926@jeremyevans.local/
>
> Jeremy also contributed the ability to use separate groups for the process
> and log files:
>
>   https://bogomips.org/unicorn-public/20180913192449.GE48926@jeremyevans.local/
>
> There's also a couple of uninteresting minor optimizations and
> documentation additions.

Otherwise, there's one extra change to use
rb_gc_register_mark_object which is finally a documented part of
the Ruby C-API, but has existed since the 1.9 days.