deps: upgrade npm to 6.6.0

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 10
• Node 8

Notable Changes

This is a pre-release of npm@6.6.0. This release marks the end of a massive internal refactor to npm that means we finally got rid of the legacy npm-registry-client in favor of the shiny, new, window.fetch-like npm-registry-fetch. Check the PR if you'd like to see the details.

• 02c837e01 #106 Make npm dist-tags the same as npm dist-tag ls. (@isaacs)
• 1065a7809 #65 Add support for IBM i. (@dmabupt)
• f58b43ef2 npm.community#3983 Report any errors above 400 as potentially not supporting audit. (@zkat)

Changelog

• 6.6.0-next.1