Bring back React as a dependency of the cocoapod
Bring back React as a dependency of the cocoapod

The pod is unable to reference RN's headers when compiled as a framework without RN as a dependency.

Fixes errors like:

```
RNMixpanel.h:9:9: 'RCTBridgeModule.h' file not found
```

davodesign84/react-native-mixpanel,davodesign84/react-native-mixpanel,davodesign84/react-native-mixpanel