Bump kubectl to version 1.13.8

Kubernetes 1.15 went GA in june, and most providers are currently on 1.13+-1.

kubectl versions should be compatible with one version behind and one ahead (1.12 and 1.14)
https://github.com/kubernetes/community/blob/master/contributors/design-proposals/release/versioning.md#supported-releases-and-component-skew

Makefile

@@ -94,7 +94,7 @@ build/kubectl test/bin/kubectl build/kustomize test/bin/kustomize build/helm tes
 cache/%/kubectl-$(KUBECTL_VERSION): docker/kubectl.version
 	mkdir -p cache/$*
 	curl --fail -L -o cache/$*/kubectl-$(KUBECTL_VERSION).tar.gz "https://dl.k8s.io/$(KUBECTL_VERSION)/kubernetes-client-$*.tar.gz"
-	[ $* != "linux-$(ARCH)" ] || echo "$(KUBECTL_CHECKSUM_$(ARCH))  cache/$*/kubectl-$(KUBECTL_VERSION).tar.gz" | shasum -a 256 -c
+	[ $* != "linux-$(ARCH)" ] || echo "$(KUBECTL_CHECKSUM_$(ARCH))  cache/$*/kubectl-$(KUBECTL_VERSION).tar.gz" | shasum -a 512 -c
 	tar -m --strip-components 3 -C ./cache/$* -xzf cache/$*/kubectl-$(KUBECTL_VERSION).tar.gz kubernetes/client/bin/kubectl
 	mv ./cache/$*/kubectl $@
 

docker/kubectl.version

@@ -1,5 +1,5 @@
-KUBECTL_VERSION=v1.11.3
-KUBECTL_CHECKSUM_amd64=14a70ac05c00fcfd7d632fc9e7a5fbc6615ce1b370bb1a0e506a24972d461493
-KUBECTL_CHECKSUM_arm=c62835797d58b50f19706d897a9106b219f5868b0a6c7bb62c6284f809c01473
-KUBECTL_CHECKSUM_arm64=a43510f821d349519ecba27b24788a0e41eae31d79bc7af73b6132190b0dcce2
+KUBECTL_VERSION=v1.13.8
+KUBECTL_CHECKSUM_amd64=b24d2f026064945a7e1571fd413bd974e165ca4514e35a3dae7df8cbf97bd283169c40e7720cbf40d6fdce2b37483df8a02a58c3a9cc9fc08aa386b43a467c0a
+KUBECTL_CHECKSUM_arm=dc60bdf00e6c7806e3c11e4f73e2ff27a603e968f22567d8c87ed5ece04263e557cb0df8d7b5196bc28a96b0b1f8ae104c503e9b9d90ce77c25e85954b54e178
+KUBECTL_CHECKSUM_arm64=00a98acd51107d1cb935cfc07ca31487290412f92ac34a91ec8c7f4b802bf798a7cc9cac22978a92001641c3c99a651c7f9a9f7f64f7cc4839a9d26021667a3b