Pyarrow version error
## Describe the bug
I installed datasets(version 1.17.0, 1.18.0, 1.18.3) but i'm right now nor able to import it because of pyarrow. when i try to import it, i get the following error:
`To use datasets, the module pyarrow>=3.0.0 is required, and the current version of pyarrow doesn't match this condition`.
i tryed with all version of pyarrow execpt `4.0.0` but still get the same error.

## Steps to reproduce the bug
```python
import datasets
```

## Expected results
A clear and concise description of the expected results.

## Actual results
AttributeError                            Traceback (most recent call last)
<ipython-input-19-652e886d387f> in <module>
----> 1 import datasets

~\AppData\Local\Continuum\anaconda3\lib\site-packages\datasets\__init__.py in <module>
     26 
     27 
---> 28 if _version.parse(pyarrow.__version__).major < 3:
     29     raise ImportWarning(
     30         "To use `datasets`, the module `pyarrow>=3.0.0` is required, and the current version of `pyarrow` doesn't match this condition.\n"

AttributeError: 'Version' object has no attribute 'major'

## Environment info
Traceback (most recent call last):
  File "c:\users\alex\appdata\local\continuum\anaconda3\lib\runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "c:\users\alex\appdata\local\continuum\anaconda3\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\Users\Alex\AppData\Local\Continuum\anaconda3\Scripts\datasets-cli.exe\__main__.py", line 5, in <module>
  File "c:\users\alex\appdata\local\continuum\anaconda3\lib\site-packages\datasets\__init__.py", line 28, in <module>
    if _version.parse(pyarrow.__version__).major < 3:
AttributeError: 'Version' object has no attribute 'major'

- `datasets` version:
- Platform: Linux(Ubuntu) and Windows: conda on the both
- Python version: 3.7
- PyArrow version: 7.0.0
