private function printUpgradeLink($io, $package)
    {
        $maxVersion = $package['direction'] === 'up' ? $package['toPretty'] : $package['fromPretty'];
        // make sure to always show a valid link, even if $maxVersion is something like dev-master
        if (!$this->isNumericVersion($maxVersion)) {
            $maxVersion = 'master';
        }
        $io->write("  https://github.com/yiisoft/yii2/blob/$maxVersion/framework/UPGRADE.md\n");
    }