Add lower bound to tqdm for tqdm.auto
Author: brendan-ai2
Pull request: yes

- It appears that `tqdm` only introduced `tqdm.auto` in 4.27.
- See https://github.com/tqdm/tqdm/releases/tag/v4.27.0.
- Without a lower bound I received an error when importing `transformers` in an environment where I already had `tqdm` installed.
- `transformers` version:
```
$ pip list | grep transformers
transformers                  2.3.0
```
- repro:
```
$ pip install tqdm==4.23
$ ipython
Python 3.6.7 |Anaconda, Inc.| (default, Oct 23 2018, 19:16:44)
Type 'copyright', 'credits' or 'license' for more information
IPython 6.4.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import transformers
---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
<ipython-input-1-279c49635b32> in <module>()
----> 1 import transformers

~/anaconda3/envs/allennlp/lib/python3.6/site-packages/transformers/__init__.py in <module>()
     18
     19 # Files and general utilities
---> 20 from .file_utils import (TRANSFORMERS_CACHE, PYTORCH_TRANSFORMERS_CACHE, PYTORCH_PRETRAINED_BERT_CACHE,
     21                          cached_path, add_start_docstrings, add_end_docstrings,
     22                          WEIGHTS_NAME, TF2_WEIGHTS_NAME, TF_WEIGHTS_NAME, CONFIG_NAME, MODEL_CARD_NAME,

~/anaconda3/envs/allennlp/lib/python3.6/site-packages/transformers/file_utils.py in <module>()
     22 from botocore.exceptions import ClientError
     23 import requests
---> 24 from tqdm.auto import tqdm
     25 from contextlib import contextmanager
     26 from . import __version__

ModuleNotFoundError: No module named 'tqdm.auto'
```

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2569?src=pr&el=h1) Report
> Merging [#2569](https://codecov.io/gh/huggingface/transformers/pull/2569?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/65a89a89768f5922e51cdc7d49990d731e3f2c03?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2569/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2569?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2569   +/-   ##
=======================================
  Coverage   74.61%   74.61%           
=======================================
  Files          87       87           
  Lines       14802    14802           
=======================================
  Hits        11044    11044           
  Misses       3758     3758
```



------

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Indeed

Thanks!