The metric Meteor doesn't work for nltk ==3.6.4
Hi @zhaowei-wang98, thanks for reporting.

We are fixing it... 
## Describe the bug
Using the metric Meteor with nltk == 3.6.4 gives a TypeError:
TypeError: descriptor 'lower' for 'str' objects doesn't apply to a 'list' object

## Steps to reproduce the bug
```python
import datasets
metric = datasets.load_metric("meteor")
predictions = ["hello world"]
references = ["hello world"]
metric.compute(predictions=predictions, references=references)
```

## Expected results
TypeError: descriptor 'lower' for 'str' objects doesn't apply to a 'list' object

I think this TypeError exists because input sentences are tokenized into lists of tokens and the str.lower() is applied to this list of tokens.

## Actual results
No error but a meteor score

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.18.3
- Platform: linux
- Python version: 3.8.12
- PyArrow version: 7.0.0
