This prepares builds on Java 9 [1].
The maven 2 tag "prerequisites" is not honored by maven 3
hence use maven-enforcer-plugin to enforce the minimum
maven version.
[1] https://issues.apache.org/jira/browse/MNG-6148
Change-Id: I57f5051a0641b1bd21f9f888f1a17d8f98e879e5
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>