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
• [x] Add standard mix format config

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.