Fixed UnicodeDecodeError: 'ascii' codec can't decode byte 0xc2
I encountered `UnicodeDecodeError: 'ascii' codec can't decode byte 0xc2 in position 3793: ordinal not in range(128)`  when running the starter example shown under the Usage section. It turned out to be related to the `load_vocab` function in `tokenization.py`. Forcing `open` to use encoding `utf8` solved this issue on my machine. 
Thanks!