Add instructions to downgrade GKE node versions

After spending too much time on installing Workflow, @kmala pointed out in Slack that I was probably running on k8s 1.3.x. Which is the default. I checked in the Console and there is no other way than to create the new cluster then downgrade it.

Hope with this (temporary) fix in the docs, other people will not make the same mistake.

Author: Wouter de Vos

src/quickstart/provider/gke/boot.md

@@ -34,6 +34,12 @@ For development and testing, we recommend you use the `n1-standard-2` machine ty
 
 Click "Create" and Google Container Engine will provision your cluster. The process will take a few minutes to complete.
 
+## Check Kubernetes version
+
+![](images/gke-node-version.jpg)]
+
+After the cluster is created, check the node version. If you are on 1.3.x, **downgrade the node version to 1.2.5**. See [Kubernetes Versions](https://deis.com/docs/workflow/installing-workflow/system-requirements/#kubernetes-versions) under System Requirements for more details.
+
 ## Install and configure the Google Cloud CLI
 
 While your container cluster is booting. You will need to install the Google Cloud CLI tools. We will use the tools to fetch cluster credentials to authenitcate to your new Kubernetes cluster.