Catch the RuntimeError raised if contentypes is not available
Catch the RuntimeError raised if contentypes is not available

In Django 1.9 it became an error to import a model of an app
before it is loaded (https://docs.djangoproject.com/en/1.9/releases/1.9/#features-removed-in-1-9)

This means the import of the user model may fail if the contenttype
or user contrib applications are not in INSTALLED_APPS.

Fixes #705

ewdurbin/raven-python,johansteffner/raven-python,ewdurbin/raven-python,akalipetis/raven-python,getsentry/raven-python,akalipetis/raven-python,akalipetis/raven-python,johansteffner/raven-python,johansteffner/raven-python,getsentry/raven-python,ewdurbin/raven-python,getsentry/raven-python