Missed retinaface_mnet025_v2.zip model file

Hi, thank you for your work!

I've faced a problem during trying to load retinaface_mnet025_v2.zip weights (i'm using insightface==0.1.5).

When I do
```model = insightface.model_zoo.get_model('retinaface_mnet025_v2')```

, i get the following error:

```
Model file is not found. Downloading.
Downloading /root/.insightface/models/retinaface_mnet025_v2.zip from http://insightface.ai/files/models/retinaface_mnet025_v2.zip...
---------------------------------------------------------------------------
RuntimeError Traceback (most recent call last)
in ()
----> 1 model = insightface.model_zoo.get_model('retinaface_mnet025_v2')

4 frames
/usr/local/lib/python3.7/dist-packages/insightface/utils/download.py in download(url, path, overwrite, sha1_hash)
68 r = requests.get(url, stream=True)
69 if r.status_code != 200:
---> 70 raise RuntimeError("Failed downloading url %s"%url)
71 total_length = r.headers.get('content-length')
72 with open(fname, 'wb') as f:

RuntimeError: Failed downloading url http://insightface.ai/files/models/retinaface_mnet025_v2.zip
```

How can I resolve this problem?

Start at insightface.model_zoo.get_model('retinaface_mnet025_v2') and trace the download request shown in the traceback to the retinaface_mnet025_v2.zip URL. Check whether the model file is available at that URL and how this model is packaged for insightface==0.1.5. Done means the model can be loaded without the download error.

Written by the indexing model from the issue text.