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

e.g.
http://www.cpantesters.org/cpan/report/1f0e7608-6bfb-1014-a820-ef492d84075e
length(undef) warns on <5.12
Dist-Zilla-Plugin-MakeMaker-Awesome-0.28

   - fix test failure caused by unstated assumption that
     Dist::Zilla::Role::TestRunner is a very recent version
   - fix uninitialized warning on 5.10.x and earlier
MIN_PERL_VERSION is harmless; leave it

as in https://github.com/rjbs/Dist-Zilla/pull/318

This bit of code was added in commit rjbs/Dist-Zilla@5c6a1202 but there
doesn't seem to be any need for it there.  We can still add a 'use' line,
while keeping this data -- which also is added to MYMETA.*.
also get the minimum perl requirement from configure, build and test prereqs

as in https://github.com/rjbs/Dist-Zilla/pull/318
Dist-Zilla-Plugin-MakeMaker-Awesome-0.29

   - retain MIN_PERL_VERSION argument to WriteMakefile()
   - extract the minimum perl requirement from all prereq phases, not just
     runtime