Updated enrollment/gender endpoint to aggregate gender counts by day.

@rocha @clintonb 
You should do another `groupby` here using `cleaned_gender` as your key. If, for some reason, the incoming data has two entries that map to unknown, only the last one will be included. Alternatively, you can can substitute `item[gender] = enrollment.count` with the block below:

```python
count = item.get(gender, 0)
count += enrollment.count
item[gender] = count
```