Elixir 1.6 - use "mix format"

[norpan]

Elixir 1.6 has a new feature called "mix format" that formats your code according to a predefined (and non-configurable) format. All of Elixir's code has already been formatted like that, and it's really great to have a standard format when merging.

So, to implement this we'd first make sure the code works for Elixir 1.6, and then check in CI that files are correctly formatted.

• https://hexdocs.pm/mix/master/Mix.Tasks.Format.html (mix format documentation)
• https://github.com/elixir-plug/plug/blob/master/.travis.yml (example of CI config)