upgrade npm in LTS to 2.15.4

[iarna]

Checklist

• tests and code linting passes
• a test and/or benchmark is included
• documentation is changed or added
• the commit message follows commit guidelines

Affected core subsystem(s)

• deps

Description of change

This update of npm LTS includes improved documentation, updated dependencies, and some important bug fixes:

• When packing/publishing, correctly exclude everything in node_modules that are not a part of bundled dependencies. Previously implicitly included files like README and Changelog found in node_modules were being included in the package tarballs.
• npm deprecate foo once again deprecates all versions of foo.
• Fixes for handling of file:// URLs on Windows.
• A fix for rmdir error handling on AIX.

Changelogs:

• v2.15.3
• v2.15.4

r: @zkat
r: @thealphanerd
r: @Fishrock123