Build the changemessage plugin on stable-2.15 and stable-2.16

Build the plugin on its own separate branches.

Change-Id: If47a9105cfbe60a68eafe5a7e63f9904ad9b81b4
diff --git a/jenkins/gerrit-bazel-plugin-changemessage.yaml b/jenkins/gerrit-bazel-plugin-changemessage.yaml
index af5eeb5..9a87e58 100644
--- a/jenkins/gerrit-bazel-plugin-changemessage.yaml
+++ b/jenkins/gerrit-bazel-plugin-changemessage.yaml
@@ -1,9 +1,9 @@
 - project:
     name: changemessage
     jobs:
-      - 'plugin-{name}-bazel-{branch}-{gerrit-branch}':
+      - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
-          gerrit-branch:
-            - master
+            - stable-2.16
+            - stable-2.15
             - stable-2.14