Update DebrilRssAtomExtension.php

I tried to read google blog http://blog.symfony.spb.ru/feeds/posts/default and I got problem because  date format is 2013-07-21T15:21:38.267+03:00. As google blogger is quite popular I think we shall add this date pattern to default array.  It's related this https://github.com/alexdebril/rss-atom-bundle/issues/21 issue.