Use `mix format` (from Elixir 1.6() instead of credo

see: https://github.com/dwyl/github-backup/pull/21#discussion_r174291676

Instead of using Credo we can use the build in Elixir mix command `mix format` https://hexdocs.pm/mix/Mix.Tasks.Format.html

However I think Heroku buildback for Elixir still use the 1.5 version so we might need to wait a bit before using 1.6
see https://github.com/HashNuke/heroku-buildpack-elixir/blob/master/elixir_buildpack.config

Start with the discussion in pull request #21 and review the referenced Heroku Elixir buildpack configuration to confirm the supported Elixir version. Check how Credo is currently used in the project, then determine whether `mix format` can replace it. Done means formatting is handled by the built-in Mix task without breaking the supported Heroku build.

Written by the indexing model from the issue text.