public static function getDifference(Release $release1, Release $release2 = null)
    {
        if ($release2 === null) {
            return self::MAJOR;
        }

        $version1 = explode('.', $release1->getVersion());
        $version2 = explode('.', $release2->getVersion());
        if (isset($version1[0]) === false || isset($version2[0]) === false || $version1[0] != $version2[0]) {
            return self::MAJOR;
        } elseif (isset($version1[1]) === false || isset($version2[1]) === false || $version1[1] != $version2[1]) {
            return self::MINOR;
        } elseif (isset($version1[2]) === false || isset($version2[2]) === false || $version1[2] != $version2[2]) {
            return self::PATCH;
        }
    }