Use setTimestamp to prevent DateTimer parse exceptions

On PHP 5.5.9 exception is thrown:

Fatal error: Uncaught exception 'Exception' with message 'DateTime::__construct(): Failed to parse time string (@) at position 0 (@): Unexpected character' in /usr/share/webapps/OpCacheGUI/src/OpCacheGUI/OpCache/Status.php:123 Stack trace: #0 /usr/share/webapps/OpCacheGUI/src/OpCacheGUI/OpCache/Status.php(123): DateTime->__construct('@') #1 /usr/share/webapps/OpCacheGUI/template/status.phtml(39): OpCacheGUI\OpCache\Status->getStatsInfo() #2 /usr/share/webapps/OpCacheGUI/bootstrap.php(95): require('/usr/share/weba...') #3 /usr/share/webapps/OpCacheGUI/public/index.php(3): require('/usr/share/weba...') #4 {main} thrown in /usr/share/webapps/OpCacheGUI/src/OpCacheGUI/OpCache/Status.php on line 123


This PR fixes at and makes the DateTime being created from timestamp in a [PHP 5.3+ way](http://php.net/manual/en/datetime.settimestamp.php).