Update .NET Core SDK

Author: RehanSaeed

appveyor.yml

@@ -2,7 +2,7 @@
 
 version: '{build}'
 
-image: Visual Studio 2017
+image: Visual Studio 2019 Preview
 install:
 - appveyor DownloadFile https://dist.nuget.org/win-x86-commandline/v4.0.0/nuget.exe
 
@@ -21,7 +21,7 @@ environment:
   DOTNET_CLI_TELEMETRY_OPTOUT: true
 
 build_script:
-- ps: choco install dotnetcore-sdk --no-progress --confirm --version 2.2.0
+- ps: choco install dotnetcore-sdk --no-progress --confirm --version 2.2.203
 - ps: .\build.ps1
 
 test: off