Update conformance-k8s.sh to 1.13

Author: smarterclayton

test/extended/conformance-k8s.sh

@@ -43,7 +43,7 @@ To recreate these results
 Nightly conformance tests are run against release branches and reported https://openshift-gce-devel.appspot.com/builds/origin-ci-test/logs/test_branch_origin_extended_conformance_k8s/
 END
 
-version="${KUBERNETES_VERSION:-release-1.12}"
+version="${KUBERNETES_VERSION:-release-1.13}"
 kubernetes="${KUBERNETES_ROOT:-${OS_ROOT}/../../../k8s.io/kubernetes}"
 if [[ -d "${kubernetes}" ]]; then
   git fetch origin --tags