moban_command check if it is False

[chfw]

https://github.com/moremoban/pypi-mobans/blob/dev/templates/travis.yml.jj2#L39

The above line would still leak moban section to downstream jobs if moban_command is None.

In other words, downstream job needs explicit 'moban_command: false' in their yaml file, which in my opinion is a burden to new starters.