deps: upgrade npm to 2.6.1

PR: #9297
PR-URL: https://github.com/joyent/node/pull/9297


Reviewed-By: Julien Gilli <julien.gilli@joyent.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>