bump global.json files to use latest version of .NET Core SDK found in VS

Ix.NET/Source/global.json

@@ -1,4 +1,3 @@
 {
-    "sources": [ ".", "tests" ]
-    //"packages": "../../packages"
+  "projects": [ ".", "tests" ]
 }

Rx.NET/Source/global.json

@@ -1,6 +1,3 @@
 {
-    "sources": [ ".", "Tests.System.Reactive" ],
-    "sdk": {
-      "version": "1.0.0-preview2-003093"
-  }
+  "projects": [ ".", "Tests.System.Reactive" ]
 }