Issue: ASSIN2 dataset checksum bug

Comment:
Using latest code, I am still facing the issue.

```python
(base) vimos@vimosmu ➜  ~ ipython
Python 3.6.7 | packaged by conda-forge | (default, Nov  6 2019, 16:19:42) 
Type 'copyright', 'credits' or 'license' for more information
IPython 7.11.1 -- An enhanced Interactive Python. Type '?' for help.

In [1]: from datasets import load_dataset

In [2]: load_dataset("assin2")
Downloading builder script: 4.24kB [00:00, 244kB/s]
Downloading metadata: 2.58kB [00:00, 2.19MB/s]
Using custom data configuration default
Downloading and preparing dataset assin2/default (download: 2.02 MiB, generated: 1.21 MiB, post-processed: Unknown size, total: 3.23 MiB) to /home/vimos/.cache/huggingface/datasets/assin2/default/1.0.0/8467f7acbda82f62ab960ca869dc1e96350e0e103a1ef7eaa43bbee530b80061...
Downloading data: 1.51MB [00:00, 102MB/s]
Downloading data: 116kB [00:00, 63.6MB/s]
Downloading data: 493kB [00:00, 95.8MB/s]                                                            
Downloading data files: 100%|██████████████████████████████████████████| 3/3 [00:00<00:00,  8.27it/s]
---------------------------------------------------------------------------
ExpectedMoreDownloadedFiles               Traceback (most recent call last)
<ipython-input-2-b367d1ffd68e> in <module>
----> 1 load_dataset("assin2")

~/anaconda3/lib/python3.6/site-packages/datasets/load.py in load_dataset(path, name, data_dir, data_files, split, cache_dir, features, download_config, download_mode, ignore_verifications, keep_in_memory, save_infos, revision, use_auth_token, task, streaming, **config_kwargs)
   1694         ignore_verifications=ignore_verifications,
   1695         try_from_hf_gcs=try_from_hf_gcs,
-> 1696         use_auth_token=use_auth_token,
   1697     )
   1698

~/anaconda3/lib/python3.6/site-packages/datasets/builder.py in download_and_prepare(self, download_config, download_mode, ignore_verifications, try_from_hf_gcs, dl_manager, base_path, use_auth_token, **download_and_prepare_kwargs)
    604                     if not downloaded_from_gcs:
    605                         self._download_and_prepare(
--> 606                             dl_manager=dl_manager, verify_infos=verify_infos, **download_and_prepare_kwargs
    607                         )
    608                     # Sync info

~/anaconda3/lib/python3.6/site-packages/datasets/builder.py in _download_and_prepare(self, dl_manager, verify_infos)
   1102
   1103     def _download_and_prepare(self, dl_manager, verify_infos):
-> 1104         super()._download_and_prepare(dl_manager, verify_infos, check_duplicate_keys=verify_infos)
   1105
   1106     def _get_examples_iterable_for_split(self, split_generator: SplitGenerator) -> ExamplesIterable:

~/anaconda3/lib/python3.6/site-packages/datasets/builder.py in _download_and_prepare(self, dl_manager, verify_infos, **prepare_split_kwargs)
    675         if verify_infos:
    676             verify_checksums(
--> 677                 self.info.download_checksums, dl_manager.get_recorded_sizes_checksums(), "dataset source files"
    678             )
    679

~/anaconda3/lib/python3.6/site-packages/datasets/utils/info_utils.py in verify_checksums(expected_checksums, recorded_checksums, verification_name)
     31         return
     32     if len(set(expected_checksums) - set(recorded_checksums)) > 0:
---> 33         raise ExpectedMoreDownloadedFiles(str(set(expected_checksums) - set(recorded_checksums)))
     34     if len(set(recorded_checksums) - set(expected_checksums)) > 0:
     35         raise UnexpectedDownloadedFile(str(set(recorded_checksums) - set(expected_checksums)))

ExpectedMoreDownloadedFiles: {'https://drive.google.com/u/0/uc?id=1kb7xq6Mb3eaqe9cOAo70BaG9ypwkIqEU&export=download', 'https://drive.google.com/u/0/uc?id=1J3FpQaHxpM-FDfBUyooh-sZF-B-bM_lU&export=download', 'https://drive.google.com/u/0/uc?id=1Q9j1a83CuKzsHCGaNulSkNxBm7Dkn7Ln&export=download'}
```