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'
```

setup.py

@@ -94,7 +94,7 @@ setup(
         # for downloading models over HTTPS
         "requests",
         # progress bars in model download and training scripts
-        "tqdm",
+        "tqdm >= 4.27",
         # for OpenAI GPT
         "regex != 2019.12.17",
         # for XLNet