Better use of spacy tokenizer in open ai and xlm tokenizers
When you do: `spacy.load('en', disable=['parser', 'tagger', 'ner', 'textcat'])` There is a high risk of throwing an exception if the user did not install the model before.

Te easiest way to use the spaCy tokenizer is the one I propose here. This way there is no need for the user to download any spaCy model.

More info here: https://spacy.io/api/tokenizer#init 
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1059?src=pr&el=h1) Report
> Merging [#1059](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1059?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/933841d903a032d93b5100220dc72db9d1283eca?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `0%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1059/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1059?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1059      +/-   ##
==========================================
- Coverage    79.6%   79.57%   -0.03%     
==========================================
  Files          42       42              
  Lines        6863     6865       +2     
==========================================
  Hits         5463     5463              
- Misses       1400     1402       +2
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1059?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_transformers/tokenization\_openai.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1059/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdG9rZW5pemF0aW9uX29wZW5haS5weQ==) | `81.51% <0%> (-0.7%)` | :arrow_down: |
| [pytorch\_transformers/tokenization\_xlm.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1059/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdG9rZW5pemF0aW9uX3hsbS5weQ==) | `83.06% <0%> (-0.68%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1059?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1059?src=pr&el=footer). Last update [933841d...388e325](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1059?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Indeed!