get_version chokes on django.db.backends.postgresql_psycopg2.base

[tstriker]

when raven tries to determine version it goes for different approaches, this get_version call here https://github.com/dcramer/raven/blob/master/raven/utils/__init__.py#L64

dies when trying to call this evil bit:
https://github.com/django/django/blob/master/django/db/backends/postgresql_psycopg2/version.py#L24

i think just wrapping it up in a try/except should suffice

thank you for the awesome tool!