Update docs to specify k8s version 1.15 or greater

Fixes tektoncd#517

Triggers now requires kubernetes version 1.15 or higher; this PR updates
the docs to reflect this.

DEVELOPMENT.md

@@ -98,7 +98,7 @@ configuring Kubernetes resources.
 Docker for Desktop using an edge version has been proven to work for both
 developing and running Pipelines. The recommended configuration is:
 
-- Kubernetes version 1.11 or later
+- Kubernetes version 1.15 or later
 - 4 vCPU nodes (`n1-standard-4`)
 - Node autoscaling, up to 3 nodes
 - API scopes for cloud-platform
@@ -110,7 +110,7 @@ To setup a cluster with GKE:
    variable (e.g. `PROJECT_ID`).
 
 1. Create a GKE cluster (with `--cluster-version=latest` but you can use any
-   version 1.11 or later):
+   version 1.15 or later):
 
    ```bash
    export PROJECT_ID=my-gcp-project

docs/install.md

@@ -14,7 +14,7 @@ Use this page to add the component to an existing Kubernetes cluster.
 
 ## Pre-requisites
 
-1. A Kubernetes cluster version 1.11 or later (_if you don't have an existing
+1. A Kubernetes cluster version 1.15 or later (_if you don't have an existing
    cluster_):
 
    ```bash