Build ref-protection plugin on stable-2.14 and stable-2.15

Change-Id: I864d8c82b2e5515ceaf94d3b6ba10c1358bf80e8
diff --git a/jenkins/gerrit-buck-bazel-plugin-ref-protection.yaml b/jenkins/gerrit-buck-bazel-plugin-ref-protection.yaml
index 1d90a4e..0e8c3b0 100644
--- a/jenkins/gerrit-buck-bazel-plugin-ref-protection.yaml
+++ b/jenkins/gerrit-buck-bazel-plugin-ref-protection.yaml
@@ -9,3 +9,5 @@
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
+            - stable-2.15
+            - stable-2.14