Fix unintentional wait if bounce is disabled

* **Summary**: If ``bounceOnCenterPanelChange`` is disabled and the center panel is still performing animations, the change of the center panel is not immediate, but waits until animations are completed.
* **Steps to Reproduce**: 
  * Create center panel with scroll view
  * Run application and flick scroll view in center panel
  * Call ``setCenterPanel`` while scroll view is still scrolling
* **Actual Results**: Center panel is changed after (unrelated) animations are finished, e.g., scroll view is still animating the scroll
* **Expected Results**: Center panel should be changed immediately
* **Cause**: ``setCenterPanel`` is using ``UIView.animateWithDuration``, but the ``animations`` block performs no changes if ``bounceOnCenterPanelChange`` is false. The ``animations`` block is thus empty and the ``completion`` block fires after unrelated animations are comleted.
* **Fix**: Don't use  ``UIView.animateWithDuration`` if ``bounceOnCenterPanelChange`` is false,  perform actions of completion block immediately.