Make scale_target_ref api_version match deployment

In an attempt to fix the following error when creating the horizontal
pod autoscaler:

the HPA controller was unable to get the target's current scale: no
matches for kind "Deployment" in group ""

From what I can tell, this is caused when the referent (in this case the
deployment) version is different to what the autoscaler expects.

https://www.terraform.io/docs/providers/kubernetes/r/horizontal_pod_autoscaler.html#arguments-10

horizontal_pod_autoscaler.tf

@@ -12,8 +12,9 @@ resource "kubernetes_horizontal_pod_autoscaler" "ambassador" {
     target_cpu_utilization_percentage = var.autoscaling_target_cpu_utilization_percentage
 
     scale_target_ref {
-      kind = "Deployment"
-      name = var.name
+      api_version = "extensions/v1beta1"
+      kind        = "Deployment"
+      name        = var.name
     }
   }