Auto npm publish

[PierrickP]

Add .travis.yml (free for open source)

language: node_js

node_js:
  - "4"
  - "6"

notifications:
  email: false

deploy:
  - provider: npm
    email: email@email.com
    api_key:
      secure: SECURED_TOKEN
    skip_cleanup: true
    on:
      branch: master
      node: "4"

node_js: i selected LTS and latest
SECURED_TOKEN: cmd travis encrypt YOUR_API_KEY (https://docs.travis-ci.com/user/deployment/npm)