use vs2017 image for appveyor
use vs2017 image for appveyor

AsynkronIT/protoactor-dotnet,raskolnikoov/protoactor-dotnet,masteryee/protoactor-dotnet,tomliversidge/protoactor-dotnet,masteryee/protoactor-dotnet,Bee-Htcpcp/protoactor-dotnet,raskolnikoov/protoactor-dotnet,tomliversidge/protoactor-dotnet,Bee-Htcpcp/protoactor-dotnet