Fix bug for removing old session on read event

The DateTime:diff function gives a DateInterval as the result.
The second attribute "s" of an DateInterval is always less than 60. So the condition "$modified->diff(new DateTime('now'))->s < $this->_lifetime" is always true once "lifetime" is greater than 60. And it is the case in most session configurations.
Merge pull request #14 from Massamba/1.4.x

Fix bug for removing old session on read event