Get Dialyxir in CI

[mmmries]

This is a first pass at #183

The goal for this PR is to get a simple dialyzer setup that works pretty quickly in CI and helps to catch any typespec errors.

[coveralls]

Coverage remained the same at 82.505% when pulling 3068b60 on mmmries:run_dialyzer_in_ci into 7bfaa23 on code-corps:master.

[mmmries]

I'm chipping away at these dialyzer errors bit by bit and I wanted to confirm something. There were a fews typespecs that were specifying a return value of {:ok, _} | {:error, Stripe.api_error_struct}. I did a little bit of digging through the git history and it looks like a while ago this library decided to wrap all those different error cases into a singular %Stipe.Error{} struct. Is that right? I switched these typespecs to specify {:error, %Stripe.Error{}} instead, but wanted to make sure that is accurate.

[mmmries]

If anyone gets a few minutes to review this, the code is now running dialyzer quickly and cleanly on CI. The changes were almost entirely just changing typespecs on things. There were several types (ie String.Types.metadata() => Stripe.Types.metadata()), a couple places where the code had been changed and the typespecs just needed to be updated (ie Api.body() being defined as map() instead of iodata()), and one place where the typespecs actually pointed out an error in the code itself (Stripe.Connect.ApplicationFee.retrieve having a default id of []).

[snewcomer]

Wow what an awesome PR! So many small errors caught (most likely by my own doing). Wish we had this earlier. 🙇