Build websession-flatfile plugin stable-2.14 and stable-2.15 branches

The websession-flatfile plugin now has its own stable-2.14 and
stable-2.15 branches.

Change-Id: Id1e9718fbb25168624de998b28b19e4da99f2612
diff --git a/jenkins/gerrit-buck-bazel-plugin-websession-flatfile.yaml b/jenkins/gerrit-buck-bazel-plugin-websession-flatfile.yaml
index c35ea23..7eb412c 100644
--- a/jenkins/gerrit-buck-bazel-plugin-websession-flatfile.yaml
+++ b/jenkins/gerrit-buck-bazel-plugin-websession-flatfile.yaml
@@ -5,11 +5,13 @@
           branch:
             - stable-2.13
             - stable-2.12
+      - 'plugin-{name}-bazel-{branch}':
+          branch:
+            - stable-2.15
+            - stable-2.14
       - 'plugin-{name}-bazel-{branch}-{gerrit-branch}':
           branch:
             - master
           gerrit-branch:
             - master
             - stable-2.16
-            - stable-2.15
-            - stable-2.14