skel: add back datanucleus enhancer plugin dependency

Current master build breaks because plugin tries to add in 3.2.6 core for DataNucleus.

This is a known issue with the DataNucleus maven setup.

See http://stackoverflow.com/questions/877949/conflicting-versions-of-datanucleus-enhancer-in-a-maven-google-app-engine-project

Several different solutions are proposed there.

We may need to revisit this issue eventually.

Testing done:

After patch, build works.

Target: master
Require-notes: no
Require-book: no
Acked-by: Paul