deps: upgrade npm to 2.3.0

* Windows improvements: no more uid is undefined errors, use `%COMSPEC%`
  when set in preference to hardcoded `cmd`, improved handling of Git
  remotes.
* Add caching based on Last-Modified / If-Modified-Since headers in
  addition to Etag-based cache validation.

PR-URL: https://github.com/iojs/io.js/pull/573


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>