Oldperl compatibility

Adding a workaround for pre-5.16 versions of Perl. 

This involves adding Sub::Current to the module's listed prereqs in dist.ini, though its use pragma isn't actually encountered unless the current Perl interpreter is < 5.16.

Per fREW's request on his blog (http://blog.afoolishmanifesto.com/archives/1758), if the current interpreter is >= 5.16, we'll continue to use the __SUB__ builtin instead.