Run tests on Node.js 8 as well

Also tried with Node.js 10 and 12, but some CLI tests explodes so awaiting
a fix for that before adding those versions explicitly.

.travis.yml

@@ -4,6 +4,7 @@ node_js:
   - 0.12
   - 4
   - 6
+  - 8
 script:
   - npm test
   - "test $TRAVIS_PULL_REQUEST != 'false' || test $TRAVIS_NODE_VERSION != '6' || npm run test-browser"