Upgrade .Net SDK to 2.2.102 to fix new test failures on AppVeyor's Ubuntu image due an image update (PowerShell#1128)

* uograde SDK to 2.2.102 to fix Ubuntu failures

* update readme

Author: bergmeister

README.md

@@ -105,7 +105,7 @@ Note: the PSScriptAnalyzer Chocolatey package is provided and supported by the c
 
 #### Requirements
 
-* [.NET Core 2.1.502 SDK](https://www.microsoft.com/net/download/dotnet-core/2.1#sdk-2.1.502) or newer patch release
+* [.NET Core 2.2.102 SDK](https://www.microsoft.com/net/download/dotnet-core/2.2#sdk-2.2.102) or newer patch release
 * [PlatyPS 0.9.0 or greater](https://github.com/PowerShell/platyPS/releases)
 * Optionally but recommended for development: [Visual Studio 2017](https://www.visualstudio.com/downloads/)
 

global.json

@@ -1,5 +1,5 @@
 {
     "sdk": {
-        "version": "2.1.502"
+        "version": "2.2.102"
     }
 }