dependencies: upgrade npm 6.12.1 -> 6.13.1

This upgrade solves the high-severity vulnerabilities regarding
https-proxy-agent that were still present in 8e6bca4.

The output of `npm audit` goes from this:
  found 29 vulnerabilities (3 low, 26 high) in 13338 scanned packages
    run `npm audit fix` to fix 4 of them.
    1 vulnerability requires semver-major dependency updates.
    24 vulnerabilities require manual review. See the full report for details.

To this:
found 5 vulnerabilities (3 low, 2 high) in 13338 scanned packages
  1 vulnerability requires semver-major dependency updates.
  4 vulnerabilities require manual review. See the full report for details.


Changelog:
- https://github.com/npm/cli/releases

6.13.1 (2019-11-18)
    BUG FIXES
    938d6124d #472 fix(fund): support funding string shorthand (@ruyadorno)
    b49c5535b #471 should not publish tap-snapshot folder (@ruyadorno)
    3471d5200 #253 Add preliminary WSL support for npm and npx (@infinnie)
    3ef295f23 #486 print quick audit report for human output (@isaacs)

    TESTING
    dbbf977ac #278 added workflow to trigger and run benchmarks (@mikemimik)
    b4f5e3825 #457 feat(docs): adding tests and updating docs to reflect changes in registry teams API. (@nomadtechie)
    454c7dd60 #456 fix git configs for git 2.23 and above (@isaacs)

    DEPENDENCIES
    661d86cd2 [email protected] (@claudiahdz)

6.13.0 (2019-11-05)
    NEW FEATURES
    4414b06d9 #273 add fund command (@ruyadorno)

    BUG FIXES
    e4455409f #281 delete ps1 files on package removal (@NoDocCat)
    cd14d4701 #279 update supported node list to remove v6.0, v6.1, v9.0 - v9.2 (@ljharb)

    DEPENDENCIES
    a37296b20 [email protected]
    d3cb3abe8 [email protected]

    TESTING
    688cd97be #272 use github actions for CI (@JasonEtco)
    9a2d8af84 #240 Clean up some flakiness and inconsistency (@isaacs)