Jenkins Oak trunk build with jdk 7???

[Julian Reschke]

Looking at

  <https://builds.apache.org/job/Apache%20Jackrabbit%20Oak%20matrix/1836/>
I wonder why we have it still configured to build with JDK 7 which by 
definition will fail :-).

Adding JDK 9 on the other hand (if possible) might be good...