updated to polyml-5.5.1;

--- a/src/Pure/ROOT.ML	Wed Sep 18 13:18:51 2013 +0200
+++ b/src/Pure/ROOT.ML	Wed Sep 18 13:31:44 2013 +0200
@@ -79,7 +79,7 @@
 then use "ML/exn_trace_polyml-5.5.1.ML"
 else ();
 
-if ML_System.name = "polyml-5.5.0"
+if ML_System.name = "polyml-5.5.0" orelse ML_System.name = "polyml-5.5.1"
 then use "ML/ml_statistics_polyml-5.5.0.ML"
 else use "ML/ml_statistics_dummy.ML";