Upgraded to use OTP24 and Rebar3

[ccapndave]

I upgraded this to use rebar3 and the new mac_* functions in OTP24. I don't know Erlang at all, only Elixir, but I was using this library and needed it to work with the latest Elixir and Erlang so I had a bash at it. It seems to work :)

[ThomasArts]

Hi @ccapndave
Bumped into this repository for a different reason and noted your PR. Good you took that on.

There is a special branch "develop-3.0" with tag 2.1.0 which already uses rebar3.
This is confusing, but for backward compatibility reasons (riak running on older Erlang versions) this cannot go into the master branch.

So the Mac changes on top of develop-3.0 branch makes more sense.
But check whether these functions support Erlang OTP 20 as well, otherwise you need to check on version and call the right one, depending on Erlang OTP version).