Django < 1.8 compatibility

[jacobwegner]

While the project has always required Django>=1.8, most of the backend has been compatible with earlier versions of Django.

Changes introduced in 148e910 (released in 0.3.0) added the max_length kwarg, which does not exist in earlier versions of Django.

To continue using the backend on earlier versions of Django, override the get_available_name method to remove the max_length kwarg:

def get_available_name(self, name):
    if self.allow_overwrite:
        return name
    return super(GoogleCloudStorage, self).get_available_name(name)