Performance improvements — upgrade Hashie, use raw_plain for document building

This is based on the discussion in #76 — it seems like given the work involved to deprecate Hashie::Mash entirely, it's worth upgrading to the new version where performance is improved.

I also pulled in @rob-at-thewebfellas commit to switch to using raw_plain with string keys for document matching.

The last change I made was pegging the Faraday version. With the current gemspec if you do a fresh install of stretcher and run specs, it fails due to a breaking API change in 0.9 version of Faraday that breaks the register_middleware call.