Fix process_failure_signal to support Celery before 2.4

I'm trying to use raven with Celery 2.2. (It's old!)

The "ExceptionInfo" structure defined in Celery <= 2.3 didn't have 'exc_info':

https://github.com/celery/celery/blob/2.3-archived/celery/datastructures.py#L169

Previously, process_failure_signal was not depend on 'exc_info':

https://github.com/getsentry/raven-python/pull/122/files

The attached patch should support both structure.
(Tested with Celery 2.2.7)