deps: Upgrade to Node v12, in setup docs and in CI.

We've been using this for local development for a little while,
and all seems well.

The major visible benefit to this upgrade is that startup times are
much improved:
  #3780 (comment)
which means everything we do from a script becomes much faster.

Fixes: #3780

.travis.yml

@@ -31,7 +31,7 @@ notifications:
     on_failure: always
 
 install:
-  - nvm install 10
+  - nvm install 12
   - node --version
   - npm i yarn -g
   - yarn

docs/howto/build-run.md

@@ -51,7 +51,7 @@ details to worry about.
 Before starting, install these dependencies if you don't have them:
 * [Git](https://git-scm.com/)
 * [Node.js](https://nodejs.org/en/download/package-manager/): use the
-  **latest 10.x** version, not 12.x or later
+  **latest 12.x** version, not 14.x or later
 * [Yarn](https://yarnpkg.com/en/docs/install), latest stable version
 
 Then, run the commands below in your terminal: