ensure that CPAN::Meta::Prereqs::merged_requirements is available

resolves
http://www.cpantesters.org/cpan/report/b6908b77-6c01-1014-815b-cd1871b9266b
Dist-Zilla-Plugin-CheckSelfDependency-0.009

          - increase minimum required version of CPAN::Meta::Prereqs (broken
            in 0.008)