[ANNOUNCE] Java 11 cron builds activated on master

[Chesnay Schepler]

Hello everyone,

I just wanted to inform everyone that we now run Java 11 builds on 
Travis as part of the cron jobs, subsuming the existing Java 9 tests. 
All existing Java 9 build/test infrastructure has been removed.

If you spot any test failures that appear to be specific to Java 11, 
please add a sub-task to FLINK-10725.

I would also encourage everyone to try out Java 11 for local development 
and usage, so that we can find pain points in the dev and user experience.