Update conditional aiohttp dependency

[macisamuele]

The goal of this PR is to make setup.py more wheel friendly (context on #13).

In case of python<3.5.3 extra_requires will ensure that aiohttp is bounded to aiohttp<3.0.
To guarantee that this approach will be effective for "old" python versions I've added a new travis task that uses python 3.5.2

NOTE: internally, this approach is already in use (ie. bravado_decorators)

[coveralls]

Coverage remained the same at 100.0% when pulling fe39301 on macisamuele:maci-update-conditional-aiohttp-dependency into ec73096 on sjaensch:master.

[coveralls]

Coverage remained the same at 100.0% when pulling fe39301 on macisamuele:maci-update-conditional-aiohttp-dependency into ec73096 on sjaensch:master.

[coveralls]

Coverage remained the same at 100.0% when pulling fe39301 on macisamuele:maci-update-conditional-aiohttp-dependency into ec73096 on sjaensch:master.

[sjaensch]

We've had issues internally with pip not installing the right version of aiohttp if bravado-asyncio was installed as a dependency of aiobravado, let's see if this way of specifying it fixes it. Otherwise we'll need to try a different approach.