[sledge] Do not disable backtrace reporting in non-debug builds

Summary:
It is desirable to programmatically enable backtrace printing in debug
mode, but in other modes it should not forcably disabled. For example,
after this diff, executing
```
$ OCAMLRUNPARAM=b sledge.trc ...
```
now prints a backtrace if an unhandled exception is raised.

Reviewed By: ngorogiannis

Differential Revision: D27564883

fbshipit-source-id: 66ea5efe2

sledge/cli/sledge_cli.ml

@@ -270,7 +270,7 @@ let readme () =
 ;;
 Memtrace.trace_if_requested ()
 ;;
-Printexc.record_backtrace Version.debug
+if Version.debug then Printexc.record_backtrace true
 ;;
 Stdlib.Sys.catch_break true