Issue: Some error inside nlp.load_dataset()

First of all, nice work!

I am going through [this overview notebook](https://colab.research.google.com/github/huggingface/nlp/blob/master/notebooks/Overview.ipynb)

In simple step `dataset = nlp.load_dataset('squad', split='validation[:10%]')`

I get an error, which is connected with some inner code, I think:
```
---------------------------------------------------------------------------

TypeError                                 Traceback (most recent call last)

<ipython-input-8-d848d3a99b8c> in <module>()
      1 # Downloading and loading a dataset
      2 
----> 3 dataset = nlp.load_dataset('squad', split='validation[:10%]')

8 frames

/usr/local/lib/python3.6/dist-packages/nlp/load.py in load_dataset(path, name, version, data_dir, data_files, split, cache_dir, download_config, download_mode, ignore_verifications, save_infos, **config_kwargs)
    515         download_mode=download_mode,
    516         ignore_verifications=ignore_verifications,
--> 517         save_infos=save_infos,
    518     )
    519 

/usr/local/lib/python3.6/dist-packages/nlp/builder.py in download_and_prepare(self, download_config, download_mode, ignore_verifications, save_infos, dl_manager, **download_and_prepare_kwargs)
    361                 verify_infos = not save_infos and not ignore_verifications
    362                 self._download_and_prepare(
--> 363                     dl_manager=dl_manager, verify_infos=verify_infos, **download_and_prepare_kwargs
    364                 )
    365                 # Sync info

/usr/local/lib/python3.6/dist-packages/nlp/builder.py in _download_and_prepare(self, dl_manager, verify_infos, **prepare_split_kwargs)
    414             try:
    415                 # Prepare split will record examples associated to the split
--> 416                 self._prepare_split(split_generator, **prepare_split_kwargs)
    417             except OSError:
    418                 raise OSError("Cannot find data file. " + (self.MANUAL_DOWNLOAD_INSTRUCTIONS or ""))

/usr/local/lib/python3.6/dist-packages/nlp/builder.py in _prepare_split(self, split_generator)
    585         fname = "{}-{}.arrow".format(self.name, split_generator.name)
    586         fpath = os.path.join(self._cache_dir, fname)
--> 587         examples_type = self.info.features.type
    588         writer = ArrowWriter(data_type=examples_type, path=fpath, writer_batch_size=self._writer_batch_size)
    589 

/usr/local/lib/python3.6/dist-packages/nlp/features.py in type(self)
    460     @property
    461     def type(self):
--> 462         return get_nested_type(self)
    463 
    464     @classmethod

/usr/local/lib/python3.6/dist-packages/nlp/features.py in get_nested_type(schema)
    370     # Nested structures: we allow dict, list/tuples, sequences
    371     if isinstance(schema, dict):
--> 372         return pa.struct({key: get_nested_type(value) for key, value in schema.items()})
    373     elif isinstance(schema, (list, tuple)):
    374         assert len(schema) == 1, "We defining list feature, you should just provide one example of the inner type"

/usr/local/lib/python3.6/dist-packages/nlp/features.py in <dictcomp>(.0)
    370     # Nested structures: we allow dict, list/tuples, sequences
    371     if isinstance(schema, dict):
--> 372         return pa.struct({key: get_nested_type(value) for key, value in schema.items()})
    373     elif isinstance(schema, (list, tuple)):
    374         assert len(schema) == 1, "We defining list feature, you should just provide one example of the inner type"

/usr/local/lib/python3.6/dist-packages/nlp/features.py in get_nested_type(schema)
    379         # We allow to reverse list of dict => dict of list for compatiblity with tfds
    380         if isinstance(inner_type, pa.StructType):
--> 381             return pa.struct(dict((f.name, pa.list_(f.type, schema.length)) for f in inner_type))
    382         return pa.list_(inner_type, schema.length)
    383 

/usr/local/lib/python3.6/dist-packages/nlp/features.py in <genexpr>(.0)
    379         # We allow to reverse list of dict => dict of list for compatiblity with tfds
    380         if isinstance(inner_type, pa.StructType):
--> 381             return pa.struct(dict((f.name, pa.list_(f.type, schema.length)) for f in inner_type))
    382         return pa.list_(inner_type, schema.length)
    383 

TypeError: list_() takes exactly one argument (2 given)
```

Comments:
Google colab has an old version of Apache Arrow built-in.
Be sure you execute the "pip install" cell and restart the notebook environment if the colab asks for it.

Thanks for reply, worked fine!