Perl 5.6.x support

This pull request *appears* to get things working for Perl 5.6.x.

My testing has been a little limited as I am lacking a working CPAN client for 5.6, so have been resorting to copying ".pm" files from my 5.18.x installation.

The Makefile.PL in the current release of Dist-CheckConflicts includes `use 5.008001`; I'm not sure of the details of how Dist::Zilla generates that file, but something may need changing there too.