Collection View flickering In Susi Chat.

This issue is due to the Custom Flow Layout (Bouncy Layout) used for Collection View.

[susiIssue.gif.zip](https://github.com/fossasia/susi_iOS/files/3025168/susiIssue.gif.zip)

https://drive.google.com/open?id=1jhkTj3OzblvTI8vZb1LetOm7l9paw5KB