[NEW] Updated Logo on Splash screen (#409)

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@RocketChat/ReactNative

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Closes #399 

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![aug-07-2018 17-39-44](https://user-images.githubusercontent.com/804994/43801415-739a0cca-9a69-11e8-8bec-d65f751e6a28.gif)
![aug-07-2018 17-31-12](https://user-images.githubusercontent.com/804994/43801416-73d19bd6-9a69-11e8-90ac-bbc7ddeed938.gif)