deps: upgrade to npm 2.14.20

Hiiii

Bit late in the week, and on the coattails of 2.14.21, but here it is anyway! v2.14.20 of npm :)

Significant changes:

• upgrades npm-install-checks and node-gyp and flattens paths a little more -- let's see how this one goes, @TheAlphaNerd!
• Some doc changes

Full release notes (including an explanation of the path length thing) are over at https://github.com/npm/npm/releases/tag/v2.14.20

r @iarna r @TheAlphaNerd r @Fishrock123 r @jasnell