use latest available .netcore sdk from appveyor

Author: kMutagene

FSharp.Plotly.sln

@@ -15,6 +15,8 @@ Project("{6EC3EE1D-3C4E-46DD-8F32-0CC8E7565705}") = "FSharp.Plotly", "src\FSharp
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "project", "project", "{BF60BC93-E09B-4E5F-9D85-95A519479D54}"
 	ProjectSection(SolutionItems) = preProject
+		.config\dotnet-tools.json = .config\dotnet-tools.json
+		global.json = global.json
 		LICENSE = LICENSE
 		README.md = README.md
 		RELEASE_NOTES.md = RELEASE_NOTES.md

global.json

@@ -1,6 +1,6 @@
 {
     "sdk": {
-      "version": "3.1.201",
+      "version": "3.1.200",
       "rollForward": "minor"
     }
   }