Update version.json

Rx.NET/Source/version.json

@@ -1,5 +1,5 @@
 {
-  "version": "4.3.0-preview.{height}",
+  "version": "4.4.0-preview.{height}",
   "publicReleaseRefSpec": [
     "^refs/heads/master$", // we release out of master
     "^refs/heads/rel/v\\d+\\.\\d+", // we also release branches starting with rel/vN.N