test(TestNotification.test_from_json): suppress deprecation warning

The test used to show the following warning:

/home/travis/build/wikimedia/pywikibot/tests/echo_tests.py:47:\
DeprecationWarning: pywikibot.echo.Notification.id is deprecated for\
6 months and 11 days; use event_id instead.

self.assertEqual(notif.id, self.data['id'])

/home/travis/build/wikimedia/pywikibot/tests/echo_tests.py:48:\
DeprecationWarning: pywikibot.echo.Notification.id is deprecated for\
6 months and 11 days; use event_id instead.

self.assertEqual(int(notif.id), notif.event_id)

Change-Id: I249340faa4a393bdb9f2d90ed79930f5a15ab3f7