also report the versions of JSON backends, to diagnose odd reports of: Subroutine JSON::XS::Boolean::(-- redefined

e.g.
http://www.cpantesters.org/cpan/report/6ce2ccda-47e5-11e4-9d2e-770f4a15a708
http://www.cpantesters.org/cpan/report/8ea08747-6c07-1014-a13e-ee746dcf0d55
http://www.cpantesters.org/cpan/report/e39a6f4c-47e3-11e4-8a5d-e6164a15a708
improve failure diagnostics