Update yarn to latest on Appveyor.

Author: iansu

Diff for: appveyor.yml

@@ -36,6 +36,9 @@ platform:
 
 install:
   - ps: Install-Product node $env:nodejs_version $env:platform
+  - ps: |
+      (New-Object Net.WebClient).DownloadFile("https://yarnpkg.com/latest.msi", "$env:temp\yarn.msi")
+      cmd /c start /wait msiexec.exe /i $env:temp\yarn.msi /quiet /qn /norestart
 
 build: off
 
@@ -46,4 +49,5 @@ skip_commits:
 test_script:
   - node --version
   - npm --version
+  - yarn --version
   - bash tasks/e2e-%test_suite%.sh