Catch ImproperlyConfigured exception for Django imports

[homm]

When I try to import rollbar in environment with installed rest_framework, I get an error:

django.core.exceptions.ImproperlyConfigured: Requested setting DATABASES, but settings are not configured. You must either define the environment variable DJANGO_SETTINGS_MODULE or call settings.configure() before accessing settings.

This is because rest_framework imports django.db.models. I fix it by catching ImproperlyConfigured in addition to ImportError.

I also catch ImproperlyConfigured for django.http.HttpRequest, but this is optional.

[sbezboro]

Thanks for the PR @homm! Looks good to me. Merged and pushed to PyPi as version 0.8.2