Update Travis CI build matrix

[jalcine]

It looks like it's failing for older versions of Erlang. That build target isn't conducive of the actual passing status.

[falood]

Thanks @jalcine , it seems it's caused by https://github.com/elixir-maru/maru/blob/master/lib/maru/supervisor.ex#L23 , will change it back to supervisor from Supervisor.init

[jalcine]

Awesome! 👍