Use yarn on appveyor

Author: geowarin

appveyor.yml

@@ -1,17 +1,19 @@
 environment:
   matrix:
-    - nodejs_version: "4"
-    - nodejs_version: "5"
-    - nodejs_version: "6"
+#    - nodejs_version: "4"
+#    - nodejs_version: "5"
+#    - nodejs_version: "6"
     - nodejs_version: "7"
 
 install:
   - ps: Install-Product node $env:nodejs_version
-  - npm install
+  - choco install yarn
+  - refreshenv
+  - yarn install
 
 test_script:
   - node --version
-  - npm --version
-  - npm test
+  - yarn --version
+  - yarn test
 
 build: off