Fix version in root pom.xml

The groupId, artifactId and version declaring a project (as opposed to a
dependency) cannot be specified via properties. The groupId and version
can be inherited from the <parent> definition, but that does not apply
here.

So let's just set the version explicitly and be done with it.

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