Use constant instead of string for Kernel events

Example:
http://symfony.com/doc/master/cookbook/event_dispatcher/event_listener.html#creating-an-event-subscriber

Currently:

        return array(
           'kernel.exception' => array(
               array('processException', 10),
               array('logException', 0),
               array('notifyException', -10),
           )
        );

I think it should be changed to:

        return array(
           KernelEvents::EXCEPTION => array(
               array('processException', 10),
               array('logException', 0),
               array('notifyException', -10),
           )
        );

If you agree you may add the "Actionable" label to this issue.

[snoek09]

I'll create a PR for this small fix.