deps: update npm to v5.5.1

Followed guide

This PR manually floats 4ca6958196 from npm upstream to update zlib. This can obviously be trumped by a 5.5.2 release if we can get it in before 9.x, but it seemed like to best option to be able to ship today and remove a bit of the pressure for next weeks release (and get 2 factor into the LTS release 🎉).

Refs: https://github.com/npm/npm/commit/4ca6958196ae41cef179473e3f7dbed9df9a32f1 Closes: https://github.com/nodejs/node/pull/16280 Fixes: https://github.com/nodejs/node/issues/14161

/cc @iarna @zkat