accomodate smaller build machine;

--- a/src/Pure/Admin/isabelle_cronjob.scala	Sat Mar 03 15:19:25 2018 +0100
+++ b/src/Pure/Admin/isabelle_cronjob.scala	Sat Mar 03 16:15:21 2018 +0100
@@ -58,8 +58,8 @@
   val build_release =
     Logger_Task("build_release", logger =>
       {
-        Isabelle_Devel.release_snapshot(rev = get_rev(), afp_rev = get_afp_rev(),
-          parallel_jobs = 4, remote_mac = "macbroy31")
+        Isabelle_Devel.release_snapshot(
+          rev = get_rev(), afp_rev = get_afp_rev(), remote_mac = "macbroy31")
       })