automate setting pom.xml <version> tag

[jeffallen]

Using the technique here: https://stackoverflow.com/a/43609273/22079 we should be able to arrange that typing "make java_deploy" chooses the correct JAR version number in the same way that "make bindist" does.

Or maybe, we'll decide that the process of publishing should all work like "make docker_push", and require that you explicitly specify BUILD_TAG.

Anyway, the change to pom.xml is the same.