Use npm v6 is the minimum required version

[ntwb]

Description

Updates the minimum required version of npm to version 6.0.0 or greater.

This is a follow up to #6610 (comment) where npm v6.0.0 was used to generate the package-lock.json file, everyone should be using the same version of npm otherwise there will be unnecessary and potentially troublesome pull requests submitted due to the differences in how npm v5 and v6 handle package-lock.json files.

How has this been tested?

Screenshots

Types of changes

Build tool update

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[gziolo]

I'm fine with the change, given that I'm already using v6.x locally :)
I think we have to update also docs if we want to introduce this change.

[ntwb]

Good call @gziolo, upon checking the docs already state to use the latest npm:

"You should also have the latest release of npm installed, npm is a separate project from Node.js and is updated frequently. If you've just installed Node.js which includes a version of npm within the installation you most likely will need to also update your npm install. To update npm, type this into your terminal: npm install npm@latest -g"

[gziolo]

npm install npm@latest -g - should we use it with Travis, too? There is no @latest part.

[gziolo]

Good docs, by the way. Future proof :)

[gziolo]

LGTM 👍

[ntwb]

npm install npm@latest -g - should we use it with Travis, too? There is no @latest part.

No, npm use @latest for new releases, for example 6.0.0 was released under latest for around two weeks before stable was bumped to 6.0.0, so let us avoid any bleeding edge releases and stick with stable

[drzraf]

npm 6 is not even provided by https://github.com/nodesource/distributions (5.6 is)
That makes use of git code much harder...

After fixing packages.json to alow npm 5.6, npm run build gave me:

ERROR in ./edit-post/components/meta-boxes/meta-boxes-panel/style.scss
Module build failed: ModuleBuildError: Module build failed: Error: Node Sass does not yet support your current environment: Linux 64-bit with Unsupported runtime (64)
For more information on which environments are supported please see:
https://github.com/sass/node-sass/releases/tag/v4.7.0

$ npm -v  
5.6.0
$ node -v
v10.1.0

which was solved by npm rebuild node-sass