Run up to 15 concurrent changes verifications

With the extra power provided by GoogleCloud, we can run many
more concurrent changes verifications.

Change-Id: I92e3a0fe2e3380027d490e4bb727c1a5733bcf9c
diff --git a/jenkins/gerrit-verifier-flow.groovy b/jenkins/gerrit-verifier-flow.groovy
index 4779d63..8455cd9 100644
--- a/jenkins/gerrit-verifier-flow.groovy
+++ b/jenkins/gerrit-verifier-flow.groovy
@@ -33,7 +33,7 @@
   static int numRetryBuilds = 3
   static int myAccountId = 1022687
   static int waitForResultTimeout = 10000
-  static int maxBuilds = 4
+  static int maxBuilds = 15
   static String verifierJobName = "Gerrit-verifier-change"
 }