tox.ini and .travis files are generated even if options are unchecked

... should not be generated if `with_travis` and `with_tox` are not checked.
Use `.diecutter-tree` for that purpose. See also [sphinx-quickstart example](https://github.com/diecutter/sphinx-quickstart/blob/master/template/docs/.diecutter-tree) and [dynamic tree documentation](https://diecutter.readthedocs.org/en/latest/client/dynamic-trees.html)
