Revert "Build: remove global install of latest npm since we want to use the paired node/npm version"

[gwwar]

The Gutenberg project intentionally uses latest npm releases. I'd still recommend avoiding this to have a little less excitement with other project releases, but happy to prep this PR if folks are okay with it.

Testing Instructions

• ✅Travis is green
• Run nvm uninstall v10.16.3
• Run ./bin/setup-local-env.sh
• We see an updated prompt to transparently run nvm install --latest-npm

$ ./bin/setup-local-env.sh 
WARNING: Node version does not match the latest long term support version. Please run this command to install and use it:
WARNING: nvm install --latest-npm
WARNING: After that, re-run the setup script to continue.

• Running this command, we see that we download the active node LTS with latest npm:
• Switching node versions will see this cached pairing.

$  nvm install --latest-npm
Found '/Users/kerryliu/checkouts/gutenberg/.nvmrc' with version <lts/*>
v10.16.3 is already installed.
Now using node v10.16.3 (npm v6.9.0)
Attempting to upgrade to the latest working version of npm...
* Installing latest `npm`; if this does not work on your node version, please report a bug!
/Users/kerryliu/.nvm/versions/node/v10.16.3/bin/npm -> /Users/kerryliu/.nvm/versions/node/v10.16.3/lib/node_modules/npm/bin/npm-cli.js
/Users/kerryliu/.nvm/versions/node/v10.16.3/bin/npx -> /Users/kerryliu/.nvm/versions/node/v10.16.3/lib/node_modules/npm/bin/npx-cli.js
+ [email protected]
added 19 packages from 13 contributors, removed 15 packages and updated 53 packages in 6.584s
* npm upgraded to: v6.11.2

Reverts #17134

[gwwar]

Looks like this is still unhappy with npm v6.11.2

So latest npm has not worked with this project for the last 3 days. I'd note that active node LTS/npm paired versions usually aren't that far behind, usually just a few minor versions.

[pento]

Someone else was able to reproduce the same problem where I reported it to the NPM team, but no info on what the problem might be:

https://npm.community/t/6-11-1-some-dependencies-are-no-longer-being-installed/9586

[ntwb]

I've discovered the regression introduced in npm 6.11.1 and added it to the discussion:

https://npm.community/t/6-11-1-some-dependencies-are-no-longer-being-installed/9586/7?u=netweb

[gwwar]

For next steps, give the build a re-run if npm tries another release with the fix in https://github.com/npm/cli/releases

Feel free to ping me if they release and folks don't have permission to re-try the travis build.

[pento]

NPM 6.11.3 looks good, no changes to package-lock.json on my machine, and Travis tests now pass.