Prevent use of function reference in config

[Ablu]

Distiller does not play nicely with such references 1. Thus, I use :erlang.make_fun for turning the {module, function, arity} tuple into a function. Since Keyword.merge would have overridden the function by the tuple again, the argument order had to be switched.

[Ablu]

The travis build seems to have failed due some outdated dependencies (too old elixir?). Locally mix test works fine (and does not test this code anyway).