travis config to test against multiple node versions but lint only once

.travis.yml

language: node_js

stages:
  - test
  - lint

cache:
  directories:
    - $HOME/.npm

jobs:
  include:
    - stage: test
      node_js: "10"
      script: npm run test:unit
    - stage: test
      node_js: "9"
      script: npm run test:unit
    - stage: test
      node_js: "8"
      script: npm run test:unit
    - stage: lint
      node_js: "10"
      script: npm run lint