fixing event (vcal) export

```
Traceback (most recent call last):
  File "/home/vagrant/.virtualenvs/wagtaildemo/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 114, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/home/vagrant/.virtualenvs/wagtaildemo/src/wagtail/wagtail/wagtailcore/views.py", line 11, in serve
    return request.site.root_page.specific.route(request, path_components)
  File "/home/vagrant/.virtualenvs/wagtaildemo/src/wagtail/wagtail/wagtailcore/models.py", line 300, in route
    return subpage.specific.route(request, remaining_components)
  File "/home/vagrant/.virtualenvs/wagtaildemo/src/wagtail/wagtail/wagtailcore/models.py", line 300, in route
    return subpage.specific.route(request, remaining_components)
  File "/home/vagrant/.virtualenvs/wagtaildemo/src/wagtail/wagtail/wagtailcore/models.py", line 305, in route
    return self.serve(request)
  File "/home/vagrant/wagtaildemo/demo/models.py", line 571, in serve
    export_event(self, 'ical'),
  File "/home/vagrant/wagtaildemo/demo/utils.py", line 32, in export_event
    start_time = datetime.time.min
AttributeError: 'method_descriptor' object has no attribute 'min'
```