travis ci yarn test

.travis.yml

@@ -7,9 +7,11 @@ node_js:
   - "4"
   - "6"
   - "stable"
+  
+install: "npm i -g yarn && yarn"
 
 script: "npm run travis"
 
 after_success:
   - npm run test:coveralls
-  - rm -rf ./coverage
+  - rm -rf ./coverage