allow tests to pass with Dist::Zilla::Role::TestRunner < 5.014

e.g.
http://www.cpantesters.org/cpan/report/9a82fcde-6cb7-1014-b70c-f5b5a694d5e7
http://www.cpantesters.org/cpan/report/f58284d3-6cb7-1014-9fea-5db7a694d5e7
Dist-Zilla-Plugin-ModuleBuildTiny-Fallback-0.007

          - fix breakage caused by unstated assumption that [ModuleBuild] is a
            very recent version