Add s3fs to requirements

Hey guys,

In case you want to read a dataframe directly from s3, for example:
```python
df = pd.read_csv('s3://my_bucket/my_object')
```

You will need an optional dependency called s3fs:
```
ImportError: Missing optional dependency 's3fs'. The s3fs package is required to handle s3 files. Use pip or conda to install s3fs.
```