Update appveyor: no more bower, use yarn.

Author: chriskrycho

appveyor.yml

@@ -21,17 +21,16 @@ install:
   - rename C:\Python27 Python27hidden
   # Typical npm stuff.
   - md C:\nc
-  - npm config set cache C:\nc
-  - npm version
-  - npm install --no-optional
-  - npm install -g bower
-  - bower install
+  - yarn --ignore-optional
+
+cache:
+ - "%LOCALAPPDATA%\\Yarn"
 
 # Post-install test scripts.
 test_script:
   # Output useful info for debugging.
-  - npm version
-  - cmd: npm test
+  - yarn versions
+  - cmd: yarn run test
 
 # Don't actually build.
 build: off