Set correct Gerrit branch for test env

Change-Id: Ib182b20f24b9ad279f3514b0e30a1415238cd60e

src/test/docker/gerrit/Dockerfile

diff --git a/src/test/docker/gerrit/Dockerfile b/src/test/docker/gerrit/Dockerfile
index 3db728b..a085d6b 100644
--- a/src/test/docker/gerrit/Dockerfile
+++ b/src/test/docker/gerrit/Dockerfile
@@ -11,7 +11,7 @@
     nfs-utils \
     && yum -y clean all
 
-ENV GERRIT_BRANCH stable-3.14
+ENV GERRIT_BRANCH stable-3.15
 
 # Add gerrit user
 RUN adduser -p -m --uid 1000 gerrit --home-dir /home/gerrit