remove unused plugins
CPAN::Meta::Merge cannot yet handle two optional_features with the same name

e.g. http://www.cpantesters.org/cpan/report/5d4d092a-5faf-11e4-adde-df4e5fa3639d

A fix exists in https://github.com/Perl-Toolchain-Gang/CPAN-Meta/pull/77
declare prereq used when AUTHOR_TESTING
it seems weird to report these backends, but Test::CPAN::Meta::* apparently may still use them *sigh*
add a doc note about setting PERL_MM_USE_DEFAULT on the author side
Dist-Zilla-Plugin-OptionalFeature-0.018

          - temporarily disable a test that relies on CPAN::Meta::Merge
            behaviour that does not yet exist