appveyor.yml - fixed the version of yarn

Author: Siddhartha

appveyor.yml

@@ -9,9 +9,11 @@ environment:
 cache:
 - "%LOCALAPPDATA%\\Yarn"
 install:
-- npm i -g yarn
+- npm i -g yarn@0.16.1
 - npm i -g typescript typings
+- yarn global add typescript typings
 - cd ".\Promact.Oauth.Server\src\Promact.Oauth.Server\"
+- yarn
 - cd..
 build_script:
 - ps: dotnet restore