fix: hardcode the version of the rxjs

`rxjs` released a new version today that is not working with the `aot` mode of [email protected]

Author: Fatme

demo-angular/package.json

@@ -25,7 +25,7 @@
     "nativescript-angular": "~6.0.6",
     "nativescript-theme-core": "~1.0.4",
     "reflect-metadata": "~0.1.10",
-    "rxjs": "~6.0.0 || >=6.1.0",
+    "rxjs": "6.3.3",
     "tns-core-modules": "4.0.1",
     "zone.js": "^0.8.26"
   },