Update global.json

Specify preview 2 in global.json in order to make the project backwards compatible for newer .Net Core tooling, especially in VS2017.

global.json

 {
-  "projects": [ "src" ]
-}
+  "projects": [ "src" ],
+  "sdk": {
\ No newline at end of file
+    "version": "1.0.0-preview2-003121"
+  }
+}