Loosen excon dependency

My app happens to use `heroku-api` which has a `excon ~> 0.16.10` dependency. Venice has a very simple reliance on excon yet depends on `~> 0.17.0`. I tested versions 0.16.10, 0.17.0, and 0.18.5 and the tests passed. In my exploration, I stumbled across [thoughtbot/appraisal](https://github.com/thoughtbot/appraisal) and threw it in to verify that the latest versions of 0.16.x, 0.17.x, and 0.18.x pass the specs.

This pull request also contains a few other very minor commits. If you'd like me to exclude some, let me know and I'll strip them out and re-push.