No longer generates a random value of SECRET_KEY if it is not set
sys.exit() if no environment SECRET_KEY and not in DEBUG mode

Context: https://github.com/doismellburning/django12factor/pull/1

Essentially, previous behaviour (uuid if SECRET_KEY not found) has
irritating consequences if you don't notice the warning logged (e.g.
invalidating authentication cookies) - lets fall back to static
SECRET_KEY if DEBUG, but fail-secure if not DEBUG (i.e. potentially
production environment)
Merge pull request #2 from doismellburning/carlio-dont-use-default-value-for-secret-key

If no environment SECRET_KEY, fail cleanly and securely if not DEBUG, ignore if DEBUG