deps: upgrade npm to 6.9.0

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Where this should land

• Node 11
• Node 10
• Node 8

Notable changes

• 2ba3a0f67 #90 Time traveling installs using the --before flag. (@zkat)
• b7b54f2d1 #3 Add support for package aliases. This allows packages to be installed under a different directory than the package name listed in package.json, and adds a new dependency type to allow this to be done for registry dependencies. (@zkat)
• 684bccf06 #146 Always save package-lock.json when using --package-lock-only. (@aeschright)
• b8b8afd40 #139 Make empty-string run-scripts run successfully as a no-op. (@vlasy)
• 8047b19b1 npm.community#3784 Match git semver ranges when flattening the tree. (@larsgw)
• e135c2bb3 npm.community#1725 Re-enable updating local packages. (@larsgw)
• 9b8b6513f npm.community#5382 npm-packlist@1.4.1: Fixes bug where core/ directories were being suddenly excluded starting in npm@6.8.0. (@zkat)

Changelogs

• 6.9.0-next.0