Bug in func. shuffle in docqa/dataset.py

Hi, Clark

Thanks for your opening source code!

I found a bug in shuffle function:

```
def shuffle_list_buckets(data, key, rng):
start = 0
end = 0
while start < len(data):
while end < len(data) and key(data[start]) == key(data[end]):
end += 1
rng.shuffle(data[start:end])
start = end
return data
```
rng.shuffle(data[start:end]) doesn't shuffle the the slice of data

Deming Ye