Working on appveyor build script

Author: kriskbx

appveyor.yml

@@ -14,7 +14,7 @@ install:
   - ps: Install-Product node $env:nodejs_version
   - set CI=true
   - npm install -g npm@latest
-  - npm install -g yarn
+  - npm install -g yarn@1.7.0
   - set PATH=%APPDATA%\npm;%PATH%
   - yarn install
 matrix: