[AUTOMATED] Update Version Pinning for Terraform to support 0.13 (#129)

## What

1. Update Version Pinning for Terraform to support 0.13

## Why

1. This is a relatively minor update that the CloudPosse module already likely supports.
1. This allows module consumers to not individually update our Terraform module to support Terraform 0.13.

Author: Gowiem

README.md

@@ -179,7 +179,7 @@ Available targets:
 
 | Name | Version |
 |------|---------|
-| terraform | ~> 0.12.0 |
+| terraform | >= 0.12.0, < 0.14.0 |
 | aws | ~> 2.0 |
 | null | ~> 2.0 |
 

docs/terraform.md

@@ -2,7 +2,7 @@
 
 | Name | Version |
 |------|---------|
-| terraform | ~> 0.12.0 |
+| terraform | >= 0.12.0, < 0.14.0 |
 | aws | ~> 2.0 |
 | null | ~> 2.0 |
 

versions.tf

@@ -1,5 +1,5 @@
 terraform {
-  required_version = "~> 0.12.0"
+  required_version = ">= 0.12.0, < 0.14.0"
 
   required_providers {
     aws  = "~> 2.0"