update version string

Rx.NET/Source/version.json

@@ -1,5 +1,5 @@
 {
-  "version": "4.0.0-preview.2.{height}",
+  "version": "4.0.0-preview.2.build.{height}",
   "publicReleaseRefSpec": [
     "^refs/heads/master$", // we release out of master
     "^refs/heads/develop$", // we release out of develop