Summary:
When call `[RCTDevMenu invalidate]`, it will cancel updateTask. But in fact, after the `[updateTask cancel]`, the updateTask's completionHandler will call `checkForUpdates` again, so it will create a new updateTask.
Closes https://github.com/facebook/react-native/pull/6165
Differential Revision: D3669576
Pulled By: javache
fbshipit-source-id: 443924b7f4be1716797fc5690e90d962cf31b923