Updated RXPTest sample to use 1.5.0-rc.1.

samples/RXPTest/package.json

@@ -30,6 +30,6 @@
     "react-dom": "^16.6.0",
     "react-native": "^0.57.3",
     "react-native-windows": "^0.57.0-rc.0",
-    "reactxp": "^1.4.0"
+    "reactxp": "^1.5.0-rc.1"
   }
 }