(ci): upgrade to use Node v10

- Node 8 was EOL in December, I upgraded locally a while ago too

- The default NPM version for Node v10 also supports `npm ci`,
  which should be a CI speed boost as well

.travis.yml

@@ -1,6 +1,6 @@
 language: node_js
 # default is the very old 0.10.48; match local version instead
-node_js: '8.9'
+node_js: '10.16.0'
 
 before_script: npm run lint
 script: npm test -- --coverage