Upgrade Zuul to just-released version 3.4.0

Change-Id: I0a0afcb4872cbf91be8bae4ea769ba71da16ba4b
diff --git a/roles/zuul_server/tasks/main.yaml b/roles/zuul_server/tasks/main.yaml
index 22fbaca..35b1d4a 100644
--- a/roles/zuul_server/tasks/main.yaml
+++ b/roles/zuul_server/tasks/main.yaml
@@ -52,7 +52,7 @@
     name: openstack.zuul
   vars:
     zuul_install_method: pip
-    zuul_git_version: '3.3.1'
+    zuul_git_version: '3.4.0'
     zuul_pip_executable: /opt/rh/rh-python36/root/bin/pip
     zuul_pip_extra_args: "--install-option='--install-scripts=/usr/local/bin'"
     zuul_file_zuul_conf_src: files/zuul/zuul.conf