move pytroch_pretrained_bert cache folder under same path as torch
This PR does two things: 

* Envs available: 
PYTORCH_PRETRAINED_BERT_CACHE > TORCH_HOME > XDG_CACHE_HOME > `~/.cache`
* If no env is set, the default path is 
`~/.cache/torch/pytorch_pretrained_bert` where `pytorch_pretrained_bert` is visible instead of hidden `.pytorch_pretrained_bert`. (since this is the cache folder, I feel it makes sense to make it visible, please correct me if I'm wrong :) 
* minor: fix typo in `hubconf.py` example.

Ok, looks good, thanks @ailzhang!