Update Jetty to version 9.4.11.v20180605
Openfire should be updated to a newer version of Jetty.
Openfire currently uses Jetty 9.2.14.v20151106
The current version of Jetty (at the time of writing) is Jetty 9.4.9.v20180320
This likely means dropping SPDY support (which I don't expect anyone will loose sleep over).