Specify MIN_PERL_VERSION, 5.8.0

5.6 is a known fail:

    Can't use an undefined value as filehandle reference at t/lib/Test/Text/FileTree.pm line 35.

Reference: http://www.cpantesters.org/cpan/report/c131a590-0e17-11e2-a5a1-15140c2f33d5