use polyml-5.5.1 (SVN), to increase chances of stability of this test;

--- a/Admin/isatest/settings/at64-poly	Mon Sep 09 14:48:16 2013 +0200
+++ b/Admin/isatest/settings/at64-poly	Mon Sep 09 15:53:02 2013 +0200
@@ -2,11 +2,11 @@
 
 init_components /home/isabelle/contrib "$HOME/admin/components/main"
 
-  POLYML_HOME="/home/polyml/polyml-5.4.1"
-  ML_SYSTEM="polyml-5.4.1"
+  POLYML_HOME="/home/polyml/polyml-svn"
+  ML_SYSTEM="polyml-5.5.1"
   ML_PLATFORM="x86_64-linux"
   ML_HOME="$POLYML_HOME/$ML_PLATFORM"
-  ML_OPTIONS="-H 1000"
+  ML_OPTIONS="-H 1000 --gcthreads 1"
 
 ISABELLE_HOME_USER=~/isabelle-at64-poly