Upgrade dependency yaml_elixir to 2.0

[Luke Taylor]

When doing mix deps.get I see the following warning

  yaml_elixir 1.4.0 RETIRED!
    (other) This version provides breaking changes and 2.0.0 should be use instead.

yaml_elixir is indeed up to 2.0 now, so maybe you want to look at upgrading. Hopefully it doesn't break anything :)

[Sebastian De Deyne]

Thanks for the heads up!

Upgraded and did some housekeeping. Tagged version 1.0.0 with the changes.