Update middleware registry for Faraday 0.9

The unified method `Faraday.register_middleware` for registering all
kinds of middleware was gone in 0.9.

https://github.com/lostisland/faraday/commit/14b458a9c971897f44d5ee3ba5354de52c998e72
Merge pull request #5 from sbadia/faraday-0.9

Update middleware registry for Faraday 0.9