Fix Duration test failure on PHP 5.3.3

Duration fails on PHP 5.3.3 due to [bug 52738](http://bugs.php.net/52738 "PHP bug #52738"): "Can't use new properties in class extended from DateInterval"

Increase minimum PHP version to [PHP 5.3.6](http://php.net/ChangeLog-5.php#5.3.6 "PHP 5.3.6 changelog") that fix bug
Merge branch 'fix-duration-5.3.3'