Library is unusable with redis older than 4.2.0 installed, even if redis isn't used

The Python redis library didn't have async support before 4.2.0, but the check for whether redis is installed doesn't take that into account: https://github.com/aio-libs/aiocache/blob/19e04665dfd1fad00c2d3ab9061a81db9acd368e/aiocache/__init__.py#L14

If `redis` is importable, the code then goes on to import `redis.asyncio`, which for older versions doesn't exist: https://github.com/aio-libs/aiocache/blob/19e04665dfd1fad00c2d3ab9061a81db9acd368e/aiocache/backends/redis.py#L5

While, technically, this isn't something that one should do, we hit this problem because of our complicated dependencies, where an internal library we use in our async code also depends on an older redis, even though that part is not used by our async code.

I think rather than hardcoding a version check, an `import redis.asyncio` in the try/except block.