Bump null-label version to support Terraform 0.13 (#94)

## what
* Bumped the pinned version of null-label to the version that supports Terraform 0.13

## why
* Module doesn't work on 0.13 without this change :)

Author: Connor Bell

main.tf

@@ -23,7 +23,7 @@ locals {
 }
 
 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"
   attributes          = var.attributes
   namespace           = var.namespace
   environment         = var.environment

nat-gateway.tf

@@ -1,5 +1,5 @@
 module "nat_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
   context    = module.label.context
   attributes = distinct(compact(concat(module.label.attributes, ["nat"])))

nat-instance.tf

@@ -1,6 +1,6 @@
 module "nat_instance_label" {
   enabled    = var.enabled
-  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"
   context    = module.label.context
   attributes = distinct(compact(concat(module.label.attributes, ["nat", "instance"])))
 }

private.tf

@@ -1,5 +1,5 @@
 module "private_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
   context    = module.label.context
   attributes = compact(concat(module.label.attributes, ["private"]))

public.tf

@@ -1,5 +1,5 @@
 module "public_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
   context    = module.label.context
   attributes = compact(concat(module.label.attributes, ["public"]))