Build 'slack-integration' master against Gerrit 'stable-3.7' branch

Change-Id: I0f37376a9b4685df31a0c8bd70e0583dc0889a8d
diff --git a/jenkins/gerrit-bazel-plugin-slack-integration.yaml b/jenkins/gerrit-bazel-plugin-slack-integration.yaml
index de84771..d8dafa5 100644
--- a/jenkins/gerrit-bazel-plugin-slack-integration.yaml
+++ b/jenkins/gerrit-bazel-plugin-slack-integration.yaml
@@ -5,6 +5,7 @@
           branch:
             - master
           gerrit-branch:
+            - stable-3.7
             - stable-3.6
       - 'plugin-{name}-bazel-{branch}':
           branch: