raindrops 0.16.0 - minor fixes and workarounds

There's mainly a fix/workaround for Ruby 2.3 now returning
locale-aware strings for File.readlink and our test suite
using strange paths allowed by *nix.

    https://bugs.ruby-lang.org/issues/12034

tcp_listener_stats won't return "true" object placeholders
if stats are configured for a non-existent listener.

There are also minor optimizations for Ruby 2.2+ (at the expense
of 2.1 and earlier).

And the usual round of minor tweaks and doc updates.

10 changes since v0.15.0:
      gemspec: avoid circular dependency on unicorn
      remove optimizations which made sense for older rubies
      linux: workaround Ruby 2.3 change
      linux: remove Pathname stdlib dependency
      add .gitattributes for Ruby method detection
      middleware: minor bytecode size reduction
      doc: update URLs and references
      README: remove indentation from URLs in RDoc
      linux: tcp_listener_stats drops "true" placeholders
      build: use '--local' domain for dev gem install