Upgrade module source version for label and route53 for 0.13 version compatibility

[ohayak]

in order to work with terraform 0.13 these sources need to be updated:

source = "git::https://github.com/cloudposse/terraform-null-label.git?ref=tags/0.16.0"
source = "git::https://github.com/cloudposse/terraform-aws-route53-cluster-hostname.git?ref=tags/0.3.0"

[jurgenweber]

I would also argue that you should increase the version constraint for the AWS provider to v3 which is the TF13 compatible version of the provider:

aws = "=> 2.0"

[jurgenweber]

cloudposse/terraform-aws-route53-cluster-hostname#23