(ci): use Node 10 Active LTS

- Node 8 Maintenance LTS is EOL in Jan 2020
- this also updates NPM to v6.9.0 (from v5.6.0)
- could add some CI perf, esp w/ new usage of `npm ci`
  - but we run in under a minute anyway, so hard to beat that!

- Cypress also seems to need Node 10 to run on Travis CI
  - https://docs.cypress.io/guides/guides/continuous-integration.html#Travis
  - https://travis-ci.org/agilgur5/physijs-webpack/builds/617388969

Author: agilgur5

.travis.yml

@@ -1,3 +1,3 @@
 language: node_js
 # default is apparently 0.10.48 for some reason O.O
-node_js: '8.9'
+node_js: '10.16.0'