Lock down Faraday to ~> 0.8.8

Faraday 0.9 changed stuff in the called env, meaning it breaks when trying to verify an email. `env[:response]` will be nil. 

it's basically the same issue encountered in VCR : https://github.com/vcr/vcr/issues/386

I've just reverted it to `~> 0.8.8` , like @samuelnp I had the issue with `json` which required some version flexibility but Faraday is fine being still on `0.8.x`.