crank up default timeout for MaSh ATP learning

--- a/src/HOL/Tools/Sledgehammer/sledgehammer_isar.ML	Wed Dec 19 22:26:26 2012 +0100
+++ b/src/HOL/Tools/Sledgehammer/sledgehammer_isar.ML	Wed Dec 19 22:43:07 2012 +0100
@@ -364,7 +364,7 @@
     (if i = 1 then "" else " " ^ string_of_int i)
   end
 
-val default_learn_prover_timeout = 0.5
+val default_learn_prover_timeout = 2.0
 
 fun hammer_away override_params subcommand opt_i fact_override state =
   let