update to yarn

.travis.yml

@@ -8,14 +8,15 @@ cache:
   yarn: true
 
 install:
-  - npm install
+  - yarn
+  - yarn add phantomjs-prebuilt
   - ./node_modules/.bin/bower install
 script:
   - ./bin/lint-features
-  - npm run-script test
-  - npm run-script test:optional-features
-  - npm run-script test:production
-  - npm run-script node-tests
+  - yarn run test
+  - yarn run test:optional-features
+  - yarn run test:production
+  - yarn run node-tests
 after_success:
   - yarn run production
   - "./bin/publish-builds"

appveyor.yml

@@ -18,15 +18,12 @@ install:
   - set path=%path%;C:\ProgramData\chocolatey\lib\PhantomJS\tools\
   # Typical npm stuff.
   - md C:\nc
-  - appveyor-retry npm install -g npm@^3
-  - appveyor-retry yarn
+  - appveyor-retry yarn install
   # Workaround https://github.com/npm/npm/wiki/Troubleshooting#upgrading-on-windows
-  - yarn version
   - yarn run bower
 # Post-install test scripts.
 test_script:
   # Output useful info for debugging.
-  - npm version
   - cmd: yarn run test
   - cmd: yarn run test:optional-features
   - cmd: yarn run test:production