Use Gerrit 3.1.5 in Dockerfile

Change-Id: Icec371ffd36f380eee42b980427f72343736a337
diff --git a/src/test/docker/gerrit/Dockerfile b/src/test/docker/gerrit/Dockerfile
index 689d2b7..bfce2c4 100644
--- a/src/test/docker/gerrit/Dockerfile
+++ b/src/test/docker/gerrit/Dockerfile
@@ -1,4 +1,4 @@
-FROM gerritcodereview/gerrit:3.1.4
+FROM gerritcodereview/gerrit:3.1.5
 
 ENV GERRIT_BRANCH=stable-3.1