What is the bare minimum required in the .travis.yml file to test an Elm project on Travis-CI?

Examples:

• https://github.com/elm-community/elm-test/blob/2.1.0/.travis.yml
• https://github.com/rtfeldman/elm-css/blob/master/.travis.yml
• https://github.com/elm-community/elm-time/blob/master/.travis.yml

The available examples include quite a lot of uncommented code ...
We need to clarify why each line exists so other people can understand the file.