Build websession-flatfile on Gerrit v2.14/15

Change-Id: I8c4bc1a211ccb7f252d42bc726271506daae6450
diff --git a/jenkins/gerrit-buck-bazel-plugin-websession-flatfile.yaml b/jenkins/gerrit-buck-bazel-plugin-websession-flatfile.yaml
index 24bfd00..ca87cb6 100644
--- a/jenkins/gerrit-buck-bazel-plugin-websession-flatfile.yaml
+++ b/jenkins/gerrit-buck-bazel-plugin-websession-flatfile.yaml
@@ -5,6 +5,10 @@
           branch:
             - stable-2.13
             - stable-2.12
-      - 'plugin-{name}-bazel-{branch}':
+      - 'plugin-{name}-bazel-{branch}-{gerrit-branch}':
           branch:
             - master
+          gerrit-branch:
+            - master
+            - stable-2.15
+            - stable-2.14