Separate auto-bumped versions clearly from the rest

In the meantime, there are a couple of scripts (SciJava scripts,
the Source Code Status job of the Jenkins server) wanting to consume
and/or modify the POM. Let's introduce a clear separator which
those scripts can respect to leave everything below to be managed
manually.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>