Move away from Class::MOP::load_class

As reported in RT#90945[0] Class::MOP::load_class is deprecated so now
we use Class::Load instead.

Thanks to Zoffix for supplying[1] this patch!

[0] https://rt.cpan.org/Public/Bug/Display.html?id=90945
[1] https://rt.cpan.org/Public/Bug/Display.html?id=90945#txn-1317403