fix build config of CI

Author: gucong3000

.travis.yml

@@ -3,6 +3,6 @@ language: node_js
 node_js:
   - node
   - '4'
-before_install: npm install --global npm
+before_install: npm install --global npm@lts
 script: npm run-script pretest && npm run-script coverage
 after_script: npx istanbul-coveralls