Use www.cpan.org as mirror

http://www.cpan.org is a global load balance DNS record that directs to
close tier-1 (fast updating) CPAN mirror servers.

http://search.cpan.org/CPAN redirects there anyway, so let's avoid the
extra re-direct
Merge pull request #330 from dagolden/topic/use-official-cpan-mirror

Use www.cpan.org as mirror