Compatibility with Devel::StackTrace 2.00

Patch provided by DAKKAR@cpan.org on [rt.cpan.org #100314]:

    Devel::StackTrace 2.00 defaults to returning strings, not refs, in the
    stack frames. This is to prevent memory leaks all over the place.

    We don't really care about leaks in a REPL, so we can just turn the
    refs back on.