SDK: switch to 2.1.500

Until we can figure out how to install on Ubuntu...

appveyor.yml

@@ -70,9 +70,8 @@ nuget:
 build_script:
 - ps: >-
     if ($env:OS -eq "Windows_NT") {
-      .\build\dotnet-install.ps1 -Version 2.1.400
+      .\build\dotnet-install.ps1 -Version 2.1.500
     }
-- sh: ./build/dotnet-install.sh --version 2.1.400
 - ps: .\build.ps1 -PullRequestNumber "$env:APPVEYOR_PULL_REQUEST_NUMBER" -CreatePackages ($env:OS -eq "Windows_NT")
 
 test: off

global.json

 
 {
   "sdk": {
-    "version": "2.1.400"
+    "version": "2.1.500"
   }
 }
\ No newline at end of file