Fix recursive object pretty-printing

This fixes a bug in PP that caused it to always use the normal #inspect
method on Objects instead of recursively applying pp_object the way it's
supposed to.

PP determines whether a given object's implementation of #inspect is the
default one by checking a regex against the return value of
method(:inspect).inspect. Rubinius's implementation of Method#inspect
doesn't match the expected format, causing PP to treat every object as
if it had a custom implementation of #inspect.

The solution is to directly check whether the method's owner is Kernel
instead of relying on the format of the inspect string.
Merge pull request #1 from rf-/fix-recursive-object-inspect

Fix recursive object pretty-printing