chore(travis): remove tests for node@6

Author: gtkatakura

.travis.yml

@@ -5,15 +5,14 @@ node_js:
   - '10'
   - '9'
   - '8'
-  - '6'
 
 before_install:
   - yarn add codecov.io coveralls
 
 after_success:
   - cat ./coverage/lcov.info | ./node_modules/codecov.io/bin/codecov.io.js
   - cat ./coverage/lcov.info | ./node_modules/coveralls/bin/coveralls.js
-  - nvm install 8 && yarn global add semantic-release
+  - yarn global add semantic-release
   - npm run travis-deploy-once "npm run semantic-release"
 
 branches: