[fix] Issue #3 resolved

[lkaybob]

• Rounded up milliseconds for DateTime::__construct

[lkaybob]

Build fails on 7.1, because implementation on microsecond is different on PHP 7.1
https://travis-ci.org/lkaybob/php-fcm-v1/builds/559820562

Should add PHP_VERSION conditional when rounding up microseconds.

if ($converted->format('u') != 0 && strpos(PHP_VERSION,'7.1') !== 0) {
    $converted = $converted->add(new DateInterval('PT1S'));
}