Update to Atmosphere 1.0.18.vaadin3 to fix regressions (#13299).

Change-Id: I028e29c4d39a032dd2a3cf24dff111ee52e50f44
Merge commit '2db2f667e1aff042ed6f3605675526f41e1e59b6'

No-op merge as master already uses a newer Atmosphere version

Change-Id: If7cf3846d149a25c2654ad99e793fb7fe037105c