adding ujson to reqirements.txt and setup.py

[oliver-zehentleitner]

You added ujson (#383) and its missing in the requirements!

[xiaods]

please upgrade it. thanks a lot.

[lancechua]

Glad to see there's some interest in using a faster JSON library.

I initially wanted it to be an optional requirement as I didn't want to add another dependency. ujson works very well since it has the same API to stdlib json for the most part.

[kimchirichie]

is this good to merge? need some verification from crowd before merging

[oliver-zehentleitner]

@kimchirichie
ujson is already used by python-binance, but its missing in setup.py and requirements.txt which is needed to create the package and to install the required dependencies... ujson itself works very well and was added to python-binance code with #383 . we also use ujson in ubwa since a couple of months....