Update MANIFEST.skip

As per Slaven's comments: I suspect the problems exists because "Makefile" is listed in MANIFEST.SKIP. Note that lines in MANIFEST.SKIP are interpreted as regexps, so "Makefile.PL" also matches and therefore it is skipped. If you want to skip "Makefile" then write something like "^Makefile$". See: https://rt.cpan.org/Ticket/Display.html?id=94317