planemo lint should check <version_command> exists

I would like `planemo lint` to check for the presence of a `` tag.

e.g. https://github.com/peterjc/pico_galaxy/issues/8

If this is missing it should at least be a warning, perhaps even an error?

We should also update https://github.com/galaxy-iuc/standards/blob/master/docs/best_practices/tool_xml.rst to make this a recommendation.