Update chapter 5 EKS example to use Kubernetes 1.15

[geerlingguy]

Yesterday 1.15 support was added to EKS: Amazon EKS now supports Kubernetes version 1.15.

This should be as simple as bumping the version number in the vars... but I need to test and verify that's the case.

[geerlingguy]

Related: aws/containers-roadmap#380

[geerlingguy]

Works:

$ kubectl get nodes
NAME                             STATUS   ROLES    AGE     VERSION
ip-172-16-103-28.ec2.internal    Ready    <none>   5m25s   v1.15.10-eks-bac369
ip-172-16-172-164.ec2.internal   Ready    <none>   5m24s   v1.15.10-eks-bac369
ip-172-16-37-223.ec2.internal    Ready    <none>   5m23s   v1.15.10-eks-bac369