Add Appraisal to test multiple Faraday version dependencies

This adds support for testing against multiple dependencies, specifically: multiple versions of Faraday (0.8 and 0.9).

Run tests across all configured versions with: `bundle exec rake appraisal`.

Currently, this pull request will fail CI since it's executing tests against Faraday 0.9. This library, currently, is broken against 0.9.  There is an existing pull request (#2) to fix that issue, as well.