add node 8 for travis test (#208)

Author: iyuq

.travis.yml

@@ -4,6 +4,7 @@ node_js:
   - "4"
   - "6"
   - "7"
+  - "8"
 script: npm run travis
 after_success:
   - cat ./coverage/coverage.json | node_modules/codecov.io/bin/codecov.io.js