Feature/elixir 0.13

I am using HTTPotion in one of my projects, hope you find this of use. These are the minimal changes needed to support Elixir 0.13-dev. You probably want to leave this until 0.13 stabilises or put it in a branch. 

Application.Behaviour.start was removed in https://github.com/elixir-lang/elixir/commit/1844aec63e84163490ac7125df1e82aa0943e264