Use yarn in travis

Author: dfcook

.travis.yml

@@ -7,8 +7,8 @@ cache:
 notifications:
   email: false
 node_js: '8'
-install: npm install
-script: npm run test
-after_success: npm run coveralls
+install: yarn
+script: yarn test
+after_success: yarn coveralls
 branches:
 only: master