planemo lint should warn about interpreter tag on version_command

Since the `interpreter` is deprecated for ``, it ought to be for `` too, and `planemo lint` should report this along side:

```
Applying linter command... CHECK
.. INFO: Command uses deprecated 'interpreter' attribute.
.. INFO: Tool contains a command with interpreter of type [python].
```

See also #574 about adding the tag to the XSD.