* Update exercises to elm-test 2.0
* Update docs to mention `elm-test` again
* Update .travis.yml to the correct version of elm-test
* Conform to the `<| \() ->` convention
Without the shebang, the buildscript might fail on environments which do
not use `bash` or a compatible shell as default.
This PR fixes the issue and makes the script more robust to the
environment.