[ci] ensure that we are using LTS versions of node

Author: gabrielcsapo

.travis.yml

@@ -6,8 +6,10 @@ script:
   - npm run coverage
   - cat coverage/lcov.info | lcov-server --upload https://lcov-server.gabrielcsapo.com
 node_js:
-  - "6"
   - "8"
+  - "10"
+  - "11"
+  - "12"
 os:
   - linux
 sudo: false