deps: upgrade npm to 2.15.11

Refs: https://github.com/nodejs/LTS/issues/143
PR-URL: https://github.com/nodejs/node/pull/9619


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>