Fix DBSecurityGroupIngress required props

http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-rds-security-group-ingress.html

You can use one of the following sets of properties:
1. CIDRIP
2. EC2-VPC: EC2SecurityGroupId
3. EC2-Classic: EC2SecurityGroupName AND EC2SecurityGroupOwnerId
Merge pull request #134 from jstrunk/patch-1

Fix DBSecurityGroupIngress required props