Build SAML plugin for Gerrit v2.13

Change-Id: Ie1e101ae870479686a69af89fbed034063cbf493
diff --git a/jenkins/gerrit-sbt-plugin-saml.yaml b/jenkins/gerrit-sbt-plugin-saml.yaml
index 890300a..f2177cd 100644
--- a/jenkins/gerrit-sbt-plugin-saml.yaml
+++ b/jenkins/gerrit-sbt-plugin-saml.yaml
@@ -3,5 +3,6 @@
     jobs:
       - 'plugin-{name}-sbt-{branch}':
           branch:
+            - stable-2.13
             - stable-2.12
             - stable-2.11