Prefer yarn over npm (#143)

* prefer yarn

* update travis

Author: tylerturdenpants

.travis.yml

@@ -3,12 +3,11 @@ node_js:
   - "8"
   - "10"
 
-cache:
-  npm: true
+cache: yarn
 
 script:
-  - npm run lint
-  - npm run test
+  - yarn lint
+  - yarn test
 
 after_success:
-- npm run coveralls
+- yarn coveralls