A few updates

[nathany-copia]

Thanks for the library. Here are a few little cleanups and updates.

• [x] fix use Mix.Config deprecation, requires Elixir 1.9+
• [x] Update dependencies
• [x] ~~Update Travis CI version to the last 2 versions~~ switch to GitHub Actions with latest Erlang/Elixir
• [x] Add standard mix format config
• [x] Fix deprecation with single-quoted strings represent charlists. Use ~c"" if you indeed want a charlist or use "" instead (I assumed these are all charlists -- please correct me if I'm wrong)
• [x] Include #129 to fix #128 in Erlang 27.1

~~I refrained from committing all the mix format changes, as that can make review difficult, and you may want to tweak line-length or other settings beforehand.~~