compatible with RN > 0.49

RN > 0.49 requires to implement requiresMainQueueSetup.
this fix and suppress that yellow box warning

```
Module RNFetchBlob requires main queue setup since it overrides `init` but doesn't implement `requiresMainQueueSetup`. In a future release React Native will default to initializing all native modules on a background thread unless explicitly opted-out of.
```

ios/RNFetchBlob/RNFetchBlob.m

@@ -42,6 +42,10 @@ + (RCTBridge *)getRCTBridge
     return rootView.bridge;
 }
 
++ (BOOL)requiresMainQueueSetup {
+    return NO;
+}
+
 RCT_EXPORT_MODULE();
 
 - (id) init {