Fix tsv read error in Windows
The initial version suffers from the error of `UnicodeDecodeError: 'charmap' codec can't decode byte 0x90 in position` when loading the `.tsv` filein **Windows** System, as indicated in https://github.com/huggingface/pytorch-pretrained-BERT/issues/52

It is solved by adding `encoding='utf-8'` when reading the `.tsv` file.

Ok, thanks @jiesutd!