Feature: Add any() helper to await first successful fulfillment of operations. (#18 by @clue)

// new: limit concurrency while awaiting any operation to complete
$promise = Queue::any(3, $urls, function ($url) use ($browser) {
    return $browser->get($url);
});

$promise->then(function (ResponseInterface $response) {
    echo 'First successful: ' . $response->getStatusCode() . PHP_EOL;
});

Minor documentation improvements (fix syntax issues and typos) and update examples. (#9 and #11 by @clue and #15 by @holtkamp)

Improve test suite to test against PHP 7.4 and PHP 7.3, drop legacy HHVM support, update distro on Travis and update project homepage. (#10 and #19 by @clue)