Allow REDIS_URL to optionally come from PaaS

This is to support a migration from Redislabs to PaaS native Redis,
allowing us to toggle between old and new using the env vars for
the instance - without needing to change the code.

app/cloudfoundry_config.py

@@ -16,6 +16,9 @@ def set_config_env_vars(vcap_services):
     # Postgres config
     os.environ['SQLALCHEMY_DATABASE_URI'] = vcap_services['postgres'][0]['credentials']['uri'].replace('postgres',
                                                                                                        'postgresql')
+    # Redis config
+    if 'redis' in vcap_services:
+        os.environ['REDIS_URL'] = vcap_services['redis'][0]['credentials']['uri']
 
     vcap_application = json.loads(os.environ['VCAP_APPLICATION'])
     os.environ['NOTIFY_ENVIRONMENT'] = vcap_application['space_name']