Bump Go to v1.13.15 in conformance pr job

Author: hidekazuna

.zuul.yaml

@@ -13,7 +13,7 @@
     run: .zuul/playbooks/run.yaml
     nodeset: ubuntu-bionic-large
     vars:
-      go_version: '1.13.8'
+      go_version: '1.13.15'
       k8s_log_dir: '{{ ansible_user_dir }}/workspace/logs/_artifacts'
       k8s_os_capi_provider_src_dir: '{{ ansible_user_dir }}/src/sigs.k8s.io/cluster-api-provider-openstack'
       extra_args: ''