[ReactNative] Fix case where scenes are not reset on gesture detaching

Summary:
Sometimes, when quicly backing out of a gesture, the scene isn't fully reset to the middle position and remains offset by a few pixels. This makes sure that never happens.

@public

Test Plan: Tested on iOS and I can no longer see the scenes stop when backing out of a gesture

Libraries/CustomComponents/Navigator/Navigator.js

@@ -874,6 +874,8 @@ var Navigator = React.createClass({
     var nextProgress = (distance - gestureDetectMovement) /
       (gesture.fullDistance - gestureDetectMovement);
     if (nextProgress < 0 && gesture.isDetachable) {
+      var gesturingToIndex = this.state.presentedIndex + this._deltaForGestureAction(this.state.activeGesture);
+      this._transitionBetween(this.state.presentedIndex, gesturingToIndex, 0);
       this._detachGesture();
       this.spring.setCurrentValue(0);
     }