[Dataset] RACE dataset Checksums error 
 Hi there, I just would like to use this awesome lib to perform a dataset fine-tuning on RACE dataset. I have performed the following steps:

```
dataset = nlp.load_dataset("race")
len(dataset["train"]), len(dataset["validation"])
```

But then I got the following error:

```
---------------------------------------------------------------------------
NonMatchingChecksumError                  Traceback (most recent call last)
<ipython-input-15-8bf7603ce0ed> in <module>
----> 1 dataset = nlp.load_dataset("race")
      2 len(dataset["train"]), len(dataset["validation"])

~/miniconda3/envs/masters/lib/python3.8/site-packages/nlp/load.py in load_dataset(path, name, version, data_dir, data_files, split, cache_dir, features, download_config, download_mode, ignore_verifications, save_infos, **config_kwargs)
    546 
    547     # Download and prepare data
--> 548     builder_instance.download_and_prepare(
    549         download_config=download_config, download_mode=download_mode, ignore_verifications=ignore_verifications,
    550     )

~/miniconda3/envs/masters/lib/python3.8/site-packages/nlp/builder.py in download_and_prepare(self, download_config, download_mode, ignore_verifications, try_from_hf_gcs, dl_manager, **download_and_prepare_kwargs)
    460                         logger.info("Dataset not on Hf google storage. Downloading and preparing it from source")
    461                 if not downloaded_from_gcs:
--> 462                     self._download_and_prepare(
    463                         dl_manager=dl_manager, verify_infos=verify_infos, **download_and_prepare_kwargs
    464                     )

~/miniconda3/envs/masters/lib/python3.8/site-packages/nlp/builder.py in _download_and_prepare(self, dl_manager, verify_infos, **prepare_split_kwargs)
    519         # Checksums verification
    520         if verify_infos:
--> 521             verify_checksums(
    522                 self.info.download_checksums, dl_manager.get_recorded_sizes_checksums(), "dataset source files"
    523             )

~/miniconda3/envs/masters/lib/python3.8/site-packages/nlp/utils/info_utils.py in verify_checksums(expected_checksums, recorded_checksums, verification_name)
     36     if len(bad_urls) > 0:
     37         error_msg = "Checksums didn't match" + for_verification_name + ":\n"
---> 38         raise NonMatchingChecksumError(error_msg + str(bad_urls))
     39     logger.info("All the checksums matched successfully" + for_verification_name)
     40 

NonMatchingChecksumError: Checksums didn't match for dataset source files:
['http://www.cs.cmu.edu/~glai1/data/race/RACE.tar.gz']
``` 
 I just cleared the cache an run it again. The error persists ):

```
 nlp (master) $ rm -rf  /Users/abarbosa/.cache/huggingface/
 nlp (master) $ python
Python 3.8.5 (default, Aug  5 2020, 03:39:04)
[Clang 10.0.0 ] :: Anaconda, Inc. on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import nlp
>>> dataset = nlp.load_dataset("race")
Downloading: 100%|██████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 4.39k/4.39k [00:00<00:00, 661kB/s]
Downloading: 100%|██████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 1.81k/1.81k [00:00<00:00, 644kB/s]
Using custom data configuration default
Downloading and preparing dataset race/default (download: 84.52 MiB, generated: 132.61 MiB, post-processed: Unknown size, total: 217.13 MiB) to /Users/abarbosa/.cache/huggingface/datasets/race/default/0.1.0/5461327f1a83549ca0d845a3159c806d2baf4f8d0d8f7d657157ce7cdf3899c2...
Downloading: 100%|██████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 25.4M/25.4M [01:03<00:00, 401kB/s]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/abarbosa/Documents/nlp/src/nlp/load.py", line 550, in load_dataset
    builder_instance.download_and_prepare(
  File "/Users/abarbosa/Documents/nlp/src/nlp/builder.py", line 471, in download_and_prepare
    self._download_and_prepare(
  File "/Users/abarbosa/Documents/nlp/src/nlp/builder.py", line 530, in _download_and_prepare
    verify_checksums(
  File "/Users/abarbosa/Documents/nlp/src/nlp/utils/info_utils.py", line 38, in verify_checksums
    raise NonMatchingChecksumError(error_msg + str(bad_urls))
nlp.utils.info_utils.NonMatchingChecksumError: Checksums didn't match for dataset source files:
['http://www.cs.cmu.edu/~glai1/data/race/RACE.tar.gz']
>>>
```