Update appveyor.yml

[smoothdeveloper]

Checking if appveyor CI fails with just updating from VS 2017 to VS 2019.

[smoothdeveloper]

Upgrading from VS2017 to VS2019 on appveyor, the build fails at the tests due to FSharp.Core version 5 in netcore2.0 app.

Failed FSharp.Data.CreateCommandTest+TraceTests.traceTimestamp
Error Message:
System.IO.FileNotFoundException : Could not load file or assembly 'FSharp.Core, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. The system cannot find the file specified.
Stack Trace:
at System.RuntimeTypeHandle.GetTypeByName(String name, Boolean throwOnError, Boolean ignoreCase, Boolean reflectionOnly, StackCrawlMarkHandle stackMark, IntPtr pPrivHostBinder, Boolean loadTypeFromPartialName, ObjectHandleOnStack type, ObjectHandleOnStack keepalive)
at System.RuntimeTypeHandle.GetTypeByName(String name, Boolean throwOnError, Boolean ignoreCase, Boolean reflectionOnly, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean loadTypeFromPartialName)
at System.RuntimeType.GetType(String typeName, Boolean throwOnError, Boolean ignoreCase, Boolean reflectionOnly, StackCrawlMark& stackMark)
at System.Type.GetType(String typeName, Boolean throwOnError)
at FSharp.Data.ISqlCommand Implementation..ctor(DesignTimeConfig cfg, FSharpChoice`3 connection, Int32 commandTimeout) in C:\projects\fsharp-data-sqlclient\src\SqlClient\ISqlCommand.fs:line 106
at FSharp.Data.CreateCommandTest.TraceTests.traceTimestamp() in C:\projects\fsharp-data-sqlclient\tests\SqlClient.Tests\CreateCommand.fs:line 124
Failed FSharp.Data.CreateCommandTest+TraceTests.traceDecimal