Also mark -1 for unstable jobs

Change-Id: I0f40e06b736c94a615b758a59d6ae0dae5f4c59e
diff --git a/jenkins/gerrit-verifier-flow.groovy b/jenkins/gerrit-verifier-flow.groovy
index 18af600..3fe5e66 100644
--- a/jenkins/gerrit-verifier-flow.groovy
+++ b/jenkins/gerrit-verifier-flow.groovy
@@ -125,6 +125,7 @@
     case Result.SUCCESS:
       return +1;
     case Result.FAILURE:
+    case Result.UNSTABLE:
       return -1;
     default:
       return 0;