khal: fix build

Upgrading icalendar to 4.0.3 broke the pinned dateutil version.
Luckily khal 0.9.10 now works with dateutil 2.7, so we no longer need to
override it.  However now one test in khal fails (the string output
changed from icalendar 4.0.2 to 4.0.3)...