plugins/reviewers: add stable-3.6 & stable-3.7 mapping

master no longer builds cleanly against these core-Gerrit versions.

Change-Id: I9013ba094d46fd57e58254d57a55d13d761e34b5
diff --git a/jenkins/gerrit-bazel-plugin-reviewers.yaml b/jenkins/gerrit-bazel-plugin-reviewers.yaml
index d329fe9..a8ba5ce 100644
--- a/jenkins/gerrit-bazel-plugin-reviewers.yaml
+++ b/jenkins/gerrit-bazel-plugin-reviewers.yaml
@@ -5,11 +5,12 @@
           extra-plugins: ''
           branch:
             - master
+            - stable-3.7
+            - stable-3.6
+
       - 'plugin-{name}-bazel-{branch}-{gerrit-branch}':
           extra-plugins: ''
           branch:
             - master
           gerrit-branch:
             - stable-3.8
-            - stable-3.7
-            - stable-3.6