Added required versions (#53)

Co-authored-by: Hidetake Iwata <int128@gmail.com>

Author: yutachaos

versions.tf

@@ -0,0 +1,9 @@
+terraform {
+  required_version = ">= 0.12.0"
+
+  required_providers {
+    aws = {
+      source  = "hashicorp/aws"
+    }
+  }
+}