(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)
- and fixes whatever bug I was getting in https://travis-ci.org/agilgur5/mst-persist/builds/613842662

.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'
 
 script: npm test -- --coverage
 after_script: