Update tokenizers to 0.8.1.rc to fix Mac OS X issues
Author: sepal
Pull request: yes

As mentioned in huggingface/tokenizers#321 tokenizers and thus transformers fails on macOS 10.11+. The issue was fixed in `0.8.1.rc2` so this PR updates the dependency to the new version.

I ran all tests on my mac with Mac OS High Sierra 10.13.6 as desribed in the [readme](https://github.com/huggingface/transformers/blob/master/README.md#tests), but not the examples: 

```
1607 passed, 304 skipped, 15294 warnings in 465.74s (0:07:45)
```



Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5867?src=pr&el=h1) Report
> Merging [#5867](https://codecov.io/gh/huggingface/transformers/pull/5867?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/ba2400189b2242620868096ae49babf93bd9ce00&el=desc) will **decrease** coverage by `0.39%`.
> The diff coverage is `n/a`.

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```diff
@@            Coverage Diff             @@
##           master    #5867      +/-   ##
==========================================
- Coverage   78.48%   78.09%   -0.40%     
==========================================
  Files         146      146              
  Lines       26200    26200              
==========================================
- Hits        20564    20461     -103     
- Misses       5636     5739     +103     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5867?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5867/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9vcGVuYWkucHk=) | `20.27% <0.00%> (-74.92%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5867/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `81.02% <0.00%> (-1.29%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5867/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `85.96% <0.00%> (-0.51%)` | :arrow_down: |
| [src/transformers/generation\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5867/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3V0aWxzLnB5) | `97.11% <0.00%> (+0.28%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5867/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `81.49% <0.00%> (+0.29%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5867/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `93.36% <0.00%> (+49.37%)` | :arrow_up: |

------

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Awesome! Thank you @sepal 

Could someone provide more detailed instructions to update the tokenizers in the version that solves the issue? I have a mac (10.13) and encountered the same problem. I tried through terminal to pip install tokenizers 0.8.1.rs2 but ended up with this error.

ERROR: Failed building wheel for tokenizers
Failed to build tokenizers
ERROR: Could not build wheels for tokenizers which use PEP 517 and cannot be installed directly