travis: Move to current LTS node.js

Version 8 is ancient and not supported any more, see
https://docs.travis-ci.com/user/languages/javascript-with-nodejs/

Newer versions of copy-webpack-plugin don't work any more with 8.

.travis.yml

@@ -2,6 +2,6 @@ dist: bionic
 sudo: false
 language: node_js
 node_js:
-  - "8"
+  - "lts/*"
 script:
   - make