Allow AWS GovCloud ARN's to validate

AWS GovCloud ARN's have a different form from regular AWS ARN's where
the ARN starts with arn:aws-us-gov instead of arn:aws [*].  Change how
getSNSApiEndpoint() validates ARN's by ignoring the second component.

* http://docs.aws.amazon.com/govcloud-us/latest/UserGuide/using-govcloud-arns.html