v1.2.0

• Feature: Use socket error codes for connection rejections
  (#17 by @clue)

  $promise = $proxy->connect('imap.example.com:143');
  $promise->then(null, function (Exception $e) {
      if ($e->getCode() === SOCKET_EACCES) {
          echo 'Failed to authenticate with proxy!';
      }
      throw $e;
  });

• Improve test suite by locking Travis distro so new defaults will not break the build and
  optionally exclude tests that rely on working internet connection
  (#15 and #16 by @clue)