Document python version required (support for async def etc.)

[nealmcb]

I get this error, presumably because async def is not in Python 3.4:

..../deepspeech/lib/python3.4/site-packages/deepspeech_server/driver/http_driver.py", line 20
    async def on_post_data(request, path):
            ^
SyntaxError: invalid syntax

The README.md and setup.py files should indicate which version of Python is required.

[MainRo]

Yes python 3.5 is the minimal required version for now. I will document this.
If this is an issue for a lot of people I may replace them with "yield/coroutine" so that python 3.4 is supported.

[nealmcb]

Cool - thanks for using the setup.py python-requires too ;)