Require at least Maven 3

As figured out in
https://github.com/knime-ip/knip-scijava-bundles/commit/6c50cdf#commitcomment-5742074,
Maven 2 has problems excluding dependencies from the bundles. But
Maven 3 works.

Thanks, Martin Horn!

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