Add mecab dependency on slow tests.
Author: mfuntowicz
Pull request: yes

Solves the following error: 

```
2020-05-19T17:01:17.3352437Z [gw0] linux -- Python 3.7.6 /home/hf/actions-r
2020-05-19T17:01:17.3354221Z     @slow
2020-05-19T17:01:17.3354825Z     def test_sequence_builders(self):
2020-05-19T17:01:17.3356512Z >       tokenizer = self.tokenizer_class.from_pretrained("bert-base-japanese-char")
2020-05-19T17:01:17.3356685Z 
2020-05-19T17:01:17.3357374Z tests/test_tokenization_bert_japanese.py:192: 
2020-05-19T17:01:17.3359012Z _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
2020-05-19T17:01:17.3360868Z .env/lib/python3.7/site-packages/transformers/tokenization_utils.py:902: in from_pretrained
2020-05-19T17:01:17.3361266Z     return cls._from_pretrained(*inputs, **kwargs)
2020-05-19T17:01:17.3363161Z .env/lib/python3.7/site-packages/transformers/tokenization_utils.py:1055: in _from_pretrained
2020-05-19T17:01:17.3363615Z     tokenizer = cls(*init_inputs, **init_kwargs)
2020-05-19T17:01:17.3365382Z .env/lib/python3.7/site-packages/transformers/tokenization_bert_japanese.py:139: in __init__
2020-05-19T17:01:17.3366229Z     do_lower_case=do_lower_case, never_split=never_split, **(mecab_kwargs or {})
2020-05-19T17:01:17.3367669Z _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
2020-05-19T17:01:17.3367895Z 
2020-05-19T17:01:17.3369474Z self = <transformers.tokenization_bert_japanese.MecabTokenizer object at 0x7f433565b9d0>
2020-05-19T17:01:17.3371043Z do_lower_case = False, never_split = None, normalize_text = True
2020-05-19T17:01:17.3371414Z mecab_option = None
2020-05-19T17:01:17.3371564Z 
2020-05-19T17:01:17.3373681Z     def __init__(self, do_lower_case=False, never_split=None, normalize_text=True, mecab_option: Optional[str] = None):
2020-05-19T17:01:17.3373909Z         """Constructs a MecabTokenizer.
2020-05-19T17:01:17.3374082Z     
2020-05-19T17:01:17.3374357Z         Args:
2020-05-19T17:01:17.3375149Z             **do_lower_case**: (`optional`) boolean (default True)
2020-05-19T17:01:17.3375850Z                 Whether to lower case the input.
2020-05-19T17:01:17.3376666Z             **never_split**: (`optional`) list of str
2020-05-19T17:01:17.3377692Z                 Kept for backward compatibility purposes.
2020-05-19T17:01:17.3378953Z                 Now implemented directly at the base class level (see :func:`PreTrainedTokenizer.tokenize`)
2020-05-19T17:01:17.3379578Z                 List of token not to split.
2020-05-19T17:01:17.3380559Z             **normalize_text**: (`optional`) boolean (default True)
2020-05-19T17:01:17.3381677Z                 Whether to apply unicode normalization to text before tokenization.
2020-05-19T17:01:17.3382985Z             **mecab_option**: (`optional`) string passed to `MeCab.Tagger` constructor (default "")
2020-05-19T17:01:17.3383398Z         """
2020-05-19T17:01:17.3384034Z         self.do_lower_case = do_lower_case
2020-05-19T17:01:17.3385088Z         self.never_split = never_split if never_split is not None else []
2020-05-19T17:01:17.3385841Z         self.normalize_text = normalize_text
2020-05-19T17:01:17.3386284Z     
2020-05-19T17:01:17.3386881Z >       import MeCab
2020-05-19T17:01:17.3388516Z E       ModuleNotFoundError: No module named 'MeCab'
```

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4481?src=pr&el=h1) Report
> Merging [#4481](https://codecov.io/gh/huggingface/transformers/pull/4481?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/384f0eb2f9d42e44094dbfd0917ccf4e6ddb462a&el=desc) will **decrease** coverage by `0.08%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4481/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4481?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4481      +/-   ##
==========================================
- Coverage   77.96%   77.88%   -0.09%     
==========================================
  Files         120      120              
  Lines       20140    20140              
==========================================
- Hits        15703    15686      -17     
- Misses       4437     4454      +17     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4481?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/4481/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `28.66% <0.00%> (-5.42%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4481?src=pr&el=footer). Last update [384f0eb...4deb915](https://codecov.io/gh/huggingface/transformers/pull/4481?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


I think 865d4d595eefc8cc9cee58fec9179bd182be0e2e might be a more "correct" way to fix this