Update tatoeba to v2021-07-22
How about this? @lhoestq @abhishekkrthakur  Hi ! I think it would be nice if people could still be able to load the old version.
Maybe this can be a parameter ? For example to load the old version they could do
```python
load_dataset("tatoeba", lang1="en", lang2="mr", date="v2020-11-09")
```

If it sounds good to you, we can add this parameter to the TatoebaConfig:
```python
class TatoebaConfig(datasets.BuilderConfig):
    def __init__(self, *args, lang1=None, lang2=None, date="v2021-07-22", **kwargs):
    self.date = date
```
and then pass the date to the URL
```python
_BASE_URL = "https://object.pouta.csc.fi/OPUS-Tatoeba/{}/moses/{}-{}.txt.zip"
```
```python
        def _base_url(lang1, lang2, date):
            return _BASE_URL.format(date, lang1, lang2)
```

What do you think ? `_DATE = "v" + "-".join(s.zfill(2) for s in _VERSION.split("."))` seems rather tricky but works well. How about this? @lhoestq 
 The CI is only failing because of the missing sections in the dataset card, and because of an issue with the CER metric that is unrelated to this PR
Tatoeba's latest version is v2021-07-22