update Readme banner image

Author: JohnEstropia

README.md

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 <p align="center">
-<img alt="CoreStore" src="https://cloud.githubusercontent.com/assets/3029684/13373932/84daee2a-ddb8-11e5-99db-fdf415620102.png" width=614 />
+<img alt="CoreStore" src="https://github.com/JohnEstropia/CoreStore/raw/develop/CoreStore.png" width=614 />
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 Unleashing the real power of Core Data with the elegance and safety of Swift