Revert "Update Travis to use undefined-sanitizer; disable failed_constexpr_test"

This reverts commit c069ae048f.

test/Jamfile.v2

@@ -56,8 +56,7 @@ system-run generic_category_test.cpp ;
 system-run system_category_test.cpp ;
 system-run after_main_test.cpp ;
 system-run failed_test.cpp ;
-compile failed_constexpr_test.cpp
-  : <toolset>gcc,<undefined-sanitizer>norecover:<build>no ;
+system-run failed_constexpr_test.cpp ;
 
 # Quick (CI) test
 run quick.cpp ;