specify the minimum perl version in metadata

closes #376.

There's also [MinimumPerl::Fast], which uses Compiler::Lexer, but it doesn't seem quite robust enough yet for primetime.

BTW I also had to add this to dist.ini to get a successful build:

    [Encoding]
    encoding = latin1
    filename = lib/App/cpanminus/fatscript.pm

However the latest release (1.7002) seems to have been built with Dist::Zilla 5, so presumably you've already hit this and fixed it.  I might be patching the wrong branch?