update rn version of demo to 0.57.8

demo/package.json

   },
   "dependencies": {
     "@babel/core": "^7.1.2",
-    "react": "16.6.0-alpha.8af6728",
-    "react-native": "0.57.3"
+    "react": "16.6.3",
+    "react-native": "0.57.8"
   },
   "devDependencies": {
     "jest": "23.6.0",
     "metro-react-native-babel-preset": "^0.48.1",
-    "react-test-renderer": "16.6.0-alpha.8af6728"
+    "react-test-renderer": "16.6.3"
   },
   "jest": {
     "preset": "react-native"