Java 9 dropping support for source/target level 1.5

[Emmanuel Bourg <ebourg@apache.org>]

This was expected but now it's effective, Java 9 no longer supports
source/target level 1.5:

http://mail.openjdk.java.net/pipermail/jdk9-dev/2014-July/000972.html

So if you update a package and see these settings please bump them to 1.6.

It might be interesting to add a Lintian warning when a jar with the
class format <= 49 is detected, that would mean the package was compiled
with a target <= 1.5 and will probably fail to build with Java 9.

Emmanuel Bourg