Further TF 0.13.0 support (#76)

For TF 0.13.0 support: https://github.com/cloudposse/terraform-null-label/releases/tag/0.17.0

main.tf

 module "label" {
-  source     = "git::https://github.com/cloudposse/terraform-null-label.git?ref=tags/0.16.0"
+  source     = "git::https://github.com/cloudposse/terraform-null-label.git?ref=tags/0.17.0"
   enabled    = var.enabled
   namespace  = var.namespace
   name       = var.name