Begin using Appraisal

Appraisal is a gem that lets us specify additional constraints for gem
development in order to test against multiple versions of dependencies.

We'll want to test this gem against Faraday 0.7 and 0.8, so I started
with those.
Partially revert 97437cb0

The rewritten test from 97437cb0 only passed with Faraday 0.8, so the
added complexity of the shared examples wasn't worth it. Fixing with
Appraisal.
Rewrite spec for both supported Faraday versions

Discovered Faraday itself has a build_url method which keeps us from
having to reference Addressable in a spec.
Merge pull request #10 from BlueKai/feature/multi_version_testing

Add appraisal; test on multiple Faraday versions

cr: @krobi64