Turns out Travis CPAN mirror is not local, add a fallback

See https://github.com/travis-ci/travis-cookbooks/commit/143a57d2ecee#diff-1

Sideffect of an empty PERL_CPANM_OPT was tickling this bug:
https://github.com/miyagawa/cpanminus/issues/288
DBI 1.628 doesn't seem to install cleanly, investigate later
Workaround for https://rt.cpan.org/Ticket/Display.html?id=87040