Update to Atmosphere 2.2 (#14534)

Does not use the new heartbeat mechanism in Atmosphere 2.2 because the heartbeat
mechanism will cause the client to reconnect every 60 seconds when idle.
It does not currently work with Vaadin because no onreopen event is fired by
Atmosphere (https://github.com/Atmosphere/atmosphere-javascript/issues/141) and thus
Vaadin will wait forever for Atmosphere to indicate that the connection is available
again after the disconnect.

Change-Id: I3a61b2c5d611f09c3092f817834e6382e1ca9894