yahns 1.9.0 - minor updates

This release improves socket inheritance support.  TCP socket
options are now applied to inherited sockets.  We also emulate
the sd_listen_fds function to allow inheriting sockets from
systemd.

HTTP status strings are now generated dynamically, allowing
applications to modify Rack::Utils::HTTP_STATUS_CODES to
apply changes in the Rack response.  Unfortunately, this leads
to minor (likely unnoticeable) performance regressions.

However, our code is not optimized for Ruby 2.2+, so users on
the latest released Ruby will benefit from reduced inline cache
and constant lookups as we reduced our constant footprint.
Expect further minor performance regressions if you are running
Ruby 2.2 and earlier.

For Ruby 2.2 users, overall performance should be largely
unchanged from 1.7.0 to 1.8.0

shortlog of changes since 1.7.0:

* use opt_str_freeze for Hash#delete
* test/helper: warn atomically
* generate response status strings dynamically
* reduce constants and optimize for Ruby 2.2+
* http_response: reduce bytecode size
* apply TCP socket options on inherited sockets
* test/test_rack_hijack.rb: try to increase test reliability
* emulate sd_listen_fds for systemd support
* test/test_rack_hijack: ensure proper ordering of log messages