set pester version (#6448)

Author: wing328

appveyor.yml

@@ -32,7 +32,7 @@ install:
   - git clone https://github.com/wing328/swagger-samples
   - ps: Start-Process -FilePath 'C:\maven\apache-maven-3.2.5\bin\mvn' -ArgumentList 'jetty:run' -WorkingDirectory "$env:appveyor_build_folder\swagger-samples\java\java-jersey-jaxrs-ci"
   - ps: $PSVersionTable.PSVersion
-  - ps: Install-Module Pester -Force -Scope CurrentUser
+  - ps: Install-Module Pester -Force -Scope CurrentUser -RequiredVersion 4.3.1
 build_script:
   - dotnet --info
   # build C# API client (netcore)