fix asyncio worker handling

simpler code to handle asyncio installation just test the version in the module.

fix #830
This won't be raised in Python 2 because of the `yield from` usages. Here's an alternative patch: https://gist.github.com/berkerpeksag/7a5091a7b7944dc9e9c2

With the new patch:

```py
>>> from gunicorn.workers import gaiohttp
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "gunicorn/workers/gaiohttp.py", line 13, in <module>
    raise RuntimeError("You need Python >= 3.3 to use the asyncio worker")
RuntimeError: You need Python >= 3.3 to use the asyncio worker
```

With #831:

```py
>>> from gunicorn.workers import gaiohttp
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "gunicorn/workers/gaiohttp.py", line 79
    yield from self.wsgi.close()
             ^
SyntaxError: invalid syntax
```

I can open a new pull request if this approach is good enough.