Add lower bound to tqdm for tqdm.auto
- It appears that `tqdm` only introduced `tqdm.auto` in 4.27.
- See https://github.com/tqdm/tqdm/releases/tag/v4.27.0.
- Without the lower bound I received the following stack trace in an environment where I already had tqdm installed:
```
File "/home/brendanr/anaconda3/envs/allennlp/lib/python3.6/site-packages/transformers/__init__.py", line 20, in <module>
from .file_utils import (TRANSFORMERS_CACHE, PYTORCH_TRANSFORMERS_CACHE, PYTORCH_PRETRAINED_BERT_CACHE,
File "/home/brendanr/anaconda3/envs/allennlp/lib/python3.6/site-packages/transformers/file_utils.py", line 24, in <module>
from tqdm.auto import tqdm
ModuleNotFoundError: No module named 'tqdm.auto'
```
