slug length counter bumps slug over max_length limit

When creating multiple videos with the same title, the unique counter appended to the slug bumps the entire thing over the max_length of 50 chars. related to #249
So the original code is sloppy and your version is better since it accounts for the possible `-100`. However I think what I'd do here instead is have it rebuild the slug every time nixing the end part every iteration so it drops exactly the right number of characters.

```
text = getattr(obj, slug_from)
base_slug = slugify(text_type(text))[:50]
for i in range(100):
    slug = base_slug[:-len(str(i))+1] + '-%s' % i
    try:
        d = {slug_field: slug}
        obj.__class__.objects.get(**d)
    except obj.__class__.DoesNotExist:
        return slug

raise ValueError('No valid slugs available for %s' % text)
```