default dzil install to cpan instead of cpanm

As of 1.7001 (Sep 08, 2013) cpanm is broken in at least these known ways on
MSWin32:

- a bug that forces system owners to perform many deletion steps when cleaning up .build manually: https://github.com/miyagawa/cpanminus/issues/370
- one bug unrelated to dzil (has to do with carton)
- a bug that causes build.log to replicate and fill up the hdd: https://github.com/miyagawa/cpanminus/pull/368

Despite the latter issue being quite urgent and a fix having been merged, miyagawa doesn't seem to show inclination to actually do a release anytime soon and hasn't communicated any intent to do one despite being asked to.

As such i consider cpanm to be unsuitable for default use by Dist::Zilla.