doc: update support status for Ruby versions
unicorn 5 will not support Ruby 1.8 anymore.
Drop mentions of Rubinius, too, it's too difficult to support due to
the proprietary and registration-required nature of its bug tracker.
The smaller memory footprint and CoW-friendly memory allocator in
mainline Ruby is a better fit for unicorn, anyways.
Since Ruby 1.9+ bundles RubyGems and gem startup is faster nowadays,
we'll just depend on that instead of not loading RubyGems.
Drop the local.mk.sample file, too, since it's way out-of-date
and probably isn't useful (I have not used it in a while).