Fix all .NET versions for AppVeyor (#333)

Author: pascalberger

.appveyor.yml

@@ -11,6 +11,9 @@ install:
   - ps: mkdir $env:DOTNET_INSTALL_DIR -Force | Out-Null
   - ps: Invoke-WebRequest -Uri "https://dotnet.microsoft.com/download/dotnet/scripts/v1/dotnet-install.ps1" -OutFile "$($env:DOTNET_INSTALL_DIR)/dotnet-install.ps1"
   - ps: '& "$($env:DOTNET_INSTALL_DIR)/dotnet-install.ps1" -Version 2.1.818 -InstallDir $env:DOTNET_INSTALL_DIR'
+  - ps: '& "$($env:DOTNET_INSTALL_DIR)/dotnet-install.ps1" -Version 3.1.414 -InstallDir $env:DOTNET_INSTALL_DIR'
+  - ps: '& "$($env:DOTNET_INSTALL_DIR)/dotnet-install.ps1" -Version 5.0.402 -InstallDir $env:DOTNET_INSTALL_DIR'
+  - ps: '& "$($env:DOTNET_INSTALL_DIR)/dotnet-install.ps1" -Version 6.0.100 -InstallDir $env:DOTNET_INSTALL_DIR'
   - ps: $env:Path = "$env:DOTNET_INSTALL_DIR;$env:Path"
   - ps: dotnet --info