deps: upgrade to npm 3.7.3

Hey! This PR supersedes #5211

This includes the following releases:

• https://github.com/npm/npm/releases/tag/v3.7.0
• https://github.com/npm/npm/releases/tag/v3.7.1
• https://github.com/npm/npm/releases/tag/v3.7.2
• https://github.com/npm/npm/releases/tag/v3.7.3

Notable changes:

• c680fa9 npmlog@2.0.2: New are-we-there-yet with performance patches from @STRML. New gauge with timer churn performance patch. This is the make-the-progress-bar-not-slow-stuff-down patch. (@iarna)
• 39dea9c #1876 (New Feature) Add support for git submodules in git remotes. (@gagern)
• 29536f4 cmd-shim@2.0.2: Final straggler using graceful-fs@<4. npm should work on master now (@ForbesLindesay)
• doc and bug fixes

r: @Fishrock123 r: @jasnell r: @mikeal r: @TheAlphaNerd r: @iarna