try to fix ci build

Author: ajyoon

.travis.yml

@@ -2,7 +2,7 @@ language: node_js
 node_js:
   - "8"
 script:
-  - npm install -g coveralls
+  - npm install --no-optional -g coveralls
   - nyc npm test && nyc report --reporter=text-lcov > coverage.lcov
 after_success:
   - cat coverage.lcov | coveralls