deps: upgrade npm to 6.8.0

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

The npm@6.8.0 release includes an implementation of RFC #10, documenting an optional field that can be used to specify the directory path for a package within a monorepo.

• 3663cdef2 #140 Update package.json docs to include repository.directory details. (@greysteil)
• 25573e9b9 npm.community#4770 Show installed but unmet peer deps. (@larsgw)

Changelog

• 6.8.0