Update Cayenne Modeler Maven plugin documentation to specify version

Update the Maven Plugin documentation at:

http://cayenne.apache.org/doc/maven2-modeler.html

to include the <version> tag. If this isn't specified, you will run the latest Cayenne Modeler in the Maven repository. This isn't good if you are using Cayenne 3.0.2, for example, and leave the version out because it will download the 3.1 Milestone and upgrade your model, leaving you with a model that no longer works with 3.0.2.