Add mix building back

[vtsatskin]

As expressed in #167, Elixir users are getting dependency issues when no mix.exs file exists specifying the required dependencies.

[benoitc]

Isn't bumping the hackney in rebar.config* version enough?

[amuino]

Is there any way I can help in getting this issue solved and a new version released to hex.pm?

[usernolan]

Also running into this issue. I manually added back the mix files to get it working, but has anyone found a rebar-only solution?

I totally understand not wanting to maintain two build config types when one should work, but also can't figure out how to get rebar to do what I want. Bumping hackney to 1.7.1 didn't work any magic for me. Let me know!

[benoitc]

applied #171 instead since rebar3 is always here and mix should use it .