Bugfix: Make iOS Slider it the responder

Summary:This fixes issues with other view (like the Navigator) stealing the responder and becoming interactive while the user is dragging the slider.

From [documentation](https://facebook.github.io/react-native/docs/gesture-responder-system.html):
- `onStartShouldSetResponder`: Does this view want to become responder on the start of a touch? -> Yes.
- `onResponderTerminationRequest`: Something else wants to become responder. Should this view release the responder? -> No.

Reviewed By: ericvicenti

Differential Revision: D3133337

fb-gh-sync-id: 3d7e1e6a2ed6fa605857cfb0549ffa71df85fd22
fbshipit-source-id: 3d7e1e6a2ed6fa605857cfb0549ffa71df85fd22

Libraries/Components/SliderIOS/SliderIOS.ios.js

@@ -131,7 +131,11 @@ var SliderIOS = React.createClass({
       onSlidingComplete && onSlidingComplete(event.nativeEvent.value);
     });
 
-    return <RCTSlider {...props}/>;
+    return <RCTSlider
+      {...props}
+      onStartShouldSetResponder={() => true}
+      onResponderTerminationRequest={() => false}
+    />;
   }
 });