fix failed test

Fixed test for the credentials check is enhanced when you run from version 0.6.2 'terraform plan'.
Ref: hashicorp/terraform#2730

Library/Formula/terraform.rb

@@ -134,34 +134,6 @@ def install
   end
 
   test do
-    minimal = testpath/"minimal.tf"
-    minimal.write <<-EOS.undent
-      variable "aws_region" {
-          default = "us-west-2"
-      }
-
-      variable "aws_amis" {
-          default = {
-              eu-west-1 = "ami-b1cf19c6"
-              us-east-1 = "ami-de7ab6b6"
-              us-west-1 = "ami-3f75767a"
-              us-west-2 = "ami-21f78e11"
-          }
-      }
-
-      # Specify the provider and access details
-      provider "aws" {
-          access_key = "this_is_a_fake_access"
-          secret_key = "this_is_a_fake_secret"
-          region = "${var.aws_region}"
-      }
-
-      resource "aws_instance" "web" {
-        instance_type = "m1.small"
-        ami = "${lookup(var.aws_amis, var.aws_region)}"
-        count = 4
-      }
-    EOS
-    system "#{bin}/terraform", "plan", "-var", "aws.region=us-west-2", testpath
+    system "#{bin}/terraform", "version"
   end
 end