fix location_id setting in commtrack signal

this shouldn't have been getting set to the location path.

also should fix http://manage.dimagi.com/default.asp?151636
Well it used to be `getattr(case, 'location_id', None)`, but now it's `getattr(case, 'location_id')` which will throw an exception if case does not have a `location_id` attribute. I think the new version is the same as `case.location_id`, unless I'm misunderstanding something.