Fix JRuby compatibility

See https://github.com/avdi/naught/pull/27

Decided to split off seperate pull requests for JRuby and RBX.

Ran into prolems identifying the line number and file of instantiation
for traceable null objects because of slight differences in
MRI's and JRuby's backtraces.

Setting the backtrace to use `Kernel.caller(3)` instead of `Kernel.caller(4)`
seems to have fixed the issue on JRuby without messing up MRI.