Fix RemovedInDjango18Warning problems

A few warning printed a lot by django 1.7 get annoying after a while.

This fixes etianen/django-watson#66

I've tried this in our project and it seems to work fine.
The my earlier commit. I tried this however unfortunately django 1.6.5 has `django.db.transaction.atomic` but it operates different to `django.db.transaction.commit_on_success`. Hence why I resorted to this rather ugly approach.