Fix for RT68781 - reports does not work with YAML

The 'YAML' module has (sometimes?) problems decoding the reports from
cpantesters - the 'reports' function in CPAN only works if you have
YAML::XS or YAML::Syck installed.

There is another issue, which is that all these modules - including
YAML - are not core modules and cpan does not warn you if you have
not installed them.

As a workaround to the YAML parsing problem, this patch downloads the
JSON version of the report. It parses this with JSON::PP - which is
in core since 5.14. Of course I understand not all of us are lucky
enough to have a relatively new perl such as that, so if you don't
have the JSON::PP module 'reports' will warn you about it and stop.