"requirements.txt" update (transformers==3.0.2)

"requirements.txt" should be updated as follows:
```
transformers==3.0.2
pytorch-lightning
torch>=1.6.0
tensorboardX
test-tube==0.7.5
nlp
rouge_score
```
Please use the following commands for using longformer:
```
pip install -r requirements.txt
python setup.py develop
```