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