Add cheshire dependency

[rfhayashi]

To make client/request* work I needed to add cheshire to my project.clj. clj-http will not parse json unless you do that. I could make a pull request to do that, but wanted to know if you think it is best to add an instruction in the readme, add it as an actual dependency, or maybe I'm missing something. Since clj-github-app explicitly uses the json parsing, I'd be inclined to add it as an actual dependency.

By the way, thank you for the library :)

[philomates]

I released this as a part of nubank/0.1.4