Route53 health check: ip_address isn't required

http://docs.aws.amazon.com/Route53/latest/APIReference/API_CreateHealthCheck.html#API_CreateHealthCheck_Requests

http://aws.amazon.com/about-aws/whats-new/2014/04/30/route-53-announces-domain-name-based-health-checks/
Merge pull request #530 from carlhoerberg/patch-1

Route53 health check: ip_address isn't required