Update to react-native 0.64 and enable Hermes

Summary:
allow-large-files

Per title. hermes enabled so that we can verify the hermes debugger works.

Reviewed By: passy

Differential Revision: D27229331

fbshipit-source-id: c2e45151b9d2097edc7e79f54eb89f9d110899d6

react-native/ReactNativeFlipperExample/App.js

@@ -85,11 +85,6 @@ const App: () => React$Node = () => {
           contentInsetAdjustmentBehavior="automatic"
           style={styles.scrollView}>
           <Header />
-          {global.HermesInternal == null ? null : (
-            <View style={styles.engine}>
-              <Text style={styles.footer}>Engine: Hermes</Text>
-            </View>
-          )}
           <View style={styles.body}>
             <View style={styles.sectionContainer}>
               <FlipperTicTacToe />