[CI] Java SE 9 has reached end of support

#1

So it is not available via apt get anymore and causes build failures on Travis recently. We’ve just merged a fix: https://github.com/theia-ide/theia/pull/5824 Please rebase your PRs to fix failed builds.

[original thread by Anton Kosyakov]

#2

https://www.oracle.com/technetwork/java/javase/downloads/jdk9-downloads-3848520.html