Changed Resque config for the staging environment: app server will connect to the Redis instance in background server using the DNS of the Elastic IP associated with the background server.

This means that, because both servers are in the same Availability Zone, the app server will actually connect to the background server using its private IP. This enables us to set a security group rules in EC2 that allow connecting to the Redis port in the Background server only from the app server.

Once these changes have been satisfactorily tested, they will be extended to the production servers.