Upgrade to kubectl v1.13

https://gitlab.com/gitlab-org/gitlab-ce/issues/52791#note_196857213

Let's start using v1.13 series of kubectl (we currently use 1.11 series). I note that the default cluster in GKE is now 1.13 and EKS is 1.13. Ideally we keep within one minor version of the server version - kubernetes only guarantees support within one minor version (https://kubernetes.io/docs/setup/release/version-skew-policy/#kubectl)

• GitLab integration - https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/kubernetes/helm.rb#L7

• AutoDevOps - https://gitlab.com/gitlab-org/cluster-integration/auto-deploy-image/tree/master

EDITED: (previous proposal was v1.12 but since then Kubernetes has released 1.16 and default versions on GKE have moved as well)