chore: Do not test with Node.js 6 any more

eslint does not support this version any more.

Author: prantlf

.travis.yml

@@ -9,7 +9,6 @@ node_js:
   - '12'
   - '10'
   - '8'
-  - '6'
 before_install:
   - npm i -g npm
 after_success: