fix templating error for manage.py

Fixes [version_control: invalid syntax near module 'six'](https://code.google.com/p/sqlalchemy-migrate/issues/detail?id=171&q=invalid)

The variable `six` was not templating nicely, it would appear as: 
```
    main(debug='False', six='<module 'six' from '/usr/lib/python2.7/dist-packages/six.pyc'>')
```

My fix is just to pop `six` from the `_var` variable which was holding `locals().copy()`.

The alternative fix to this would be to print the string with escaped quotes so we don't have malformed strings.