Downloading dataset error with pyarrow.lib.RecordBatch 
 I am trying to download `sentiment140` and I have the following error

```
/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)
    518         download_mode=download_mode,
    519         ignore_verifications=ignore_verifications,
--> 520         save_infos=save_infos,
    521     )
    522 

/usr/local/lib/python3.6/dist-packages/nlp/builder.py in download_and_prepare(self, download_config, download_mode, ignore_verifications, save_infos, try_from_hf_gcs, dl_manager, **download_and_prepare_kwargs)
    418                 verify_infos = not save_infos and not ignore_verifications
    419                 self._download_and_prepare(
--> 420                     dl_manager=dl_manager, verify_infos=verify_infos, **download_and_prepare_kwargs
    421                 )
    422                 # Sync info

/usr/local/lib/python3.6/dist-packages/nlp/builder.py in _download_and_prepare(self, dl_manager, verify_infos, **prepare_split_kwargs)
    472             try:
    473                 # Prepare split will record examples associated to the split
--> 474                 self._prepare_split(split_generator, **prepare_split_kwargs)
    475             except OSError:
    476                 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)
    652         for key, record in utils.tqdm(generator, unit=" examples", total=split_info.num_examples, leave=False):
    653             example = self.info.features.encode_example(record)
--> 654             writer.write(example)
    655         num_examples, num_bytes = writer.finalize()
    656 

/usr/local/lib/python3.6/dist-packages/nlp/arrow_writer.py in write(self, example, writer_batch_size)
    143             self._build_writer(pa_table=pa.Table.from_pydict(example))
    144         if writer_batch_size is not None and len(self.current_rows) >= writer_batch_size:
--> 145             self.write_on_file()
    146 
    147     def write_batch(

/usr/local/lib/python3.6/dist-packages/nlp/arrow_writer.py in write_on_file(self)
    127             else:
    128                 # All good
--> 129                 self._write_array_on_file(pa_array)
    130             self.current_rows = []
    131 

/usr/local/lib/python3.6/dist-packages/nlp/arrow_writer.py in _write_array_on_file(self, pa_array)
     96     def _write_array_on_file(self, pa_array):
     97         """Write a PyArrow Array"""
---> 98         pa_batch = pa.RecordBatch.from_struct_array(pa_array)
     99         self._num_bytes += pa_array.nbytes
    100         self.pa_writer.write_batch(pa_batch)

AttributeError: type object 'pyarrow.lib.RecordBatch' has no attribute 'from_struct_array'
```

I installed the last version and ran the following command:

```python
import nlp
sentiment140 = nlp.load_dataset('sentiment140', cache_dir='/content')
``` 
 When you install `nlp` for the first time on a Colab runtime, it updates the `pyarrow` library that was already on colab. This update shows this message on colab:
```
WARNING: The following packages were previously imported in this runtime:
  [pyarrow]
You must restart the runtime in order to use newly installed versions.
```
You just have to restart the runtime and it should be fine.
If you don't restart, then it breaks like in your message.