Rewrite Ruby parts of Java exceptions raised from JI calls.

This flag was added for #1313 but not turned on. Due to continued
reports from people having trouble interpreting JI stack traces,
we have decided to do this transformation by default.

Specifying -Xrewrite.java.trace=false or
-Djruby.rewrite.java.trace=false will return to the old non-
rewriting behvaior.

See #1504 and #1313 at least. Fixes #1504.