Cannot import DistilBert classes
Tried installing from the master, and couldn't do it. 
![image](https://user-images.githubusercontent.com/347398/63883240-91c73e80-c988-11e9-9a22-241ac48479c3.png)

My bad. Forgot to ignore pip cache
```
pip install git+https://github.com/huggingface/pytorch-transformers --no-cache-dir
```