Put SDK ver in global.json for vs 2015 support

Ix.NET/Source/global.json

@@ -1,3 +1,6 @@
 {
-  "projects": [ ".", "tests" ]
+  "projects": [ ".", "tests" ],
+  "sdk": {
+      "version": "1.0.0-preview2-003131"
+  }
 }

Rx.NET/Source/global.json

@@ -1,3 +1,6 @@
 {
-  "projects": [ ".", "Tests.System.Reactive" ]
+  "projects": [ ".", "Tests.System.Reactive" ],
+  "sdk": {
+      "version": "1.0.0-preview2-003131"
+  }
 }