Update middleware registration for Faraday 0.9.

Faraday.register_middleware was removed in 0.9.

https://github.com/lostisland/faraday/commit/14b458a9c971897f44d5ee3ba5354de52c998e72.

Resolves https://github.com/mosaicxm/faraday_middleware-rss/issues/3.