v2.3.0

Ruby posix_mq 2.3.0 - updates for Ruby 2.3+

This release updates our POSIX_MQ#close to match the idempotent
IO#close behavior under Ruby 2.3 and later.  Previous Ruby versions
continue to be supported, but POSIX_MQ#close remains non-idempotent
when built against old Ruby versions.

4 changes since v2.2.0:

      enable frozen_string_literal for Ruby 2.3+
      support idempotent POSIX_MQ#close
      README: remove cgit and ssoma references
      build: use '--local' domain for dev gem install