Fix build on MacOS

[dvic]

Rustler generates this .cargo/config.toml for MacOS: https://github.com/rusterlium/rustler/pull/192

[dvic]

FYI: I tested this locally both using rebar3 hex build and in a mix project.

[dvic]

By the way: I noticed that the Elixir test uses :rocker.open_default/1 but this function does not exist? Shouldn't it be :rocker.open/1? However, I do see in the erlang test one left-over as well: https://github.com/Vonmo/rocker/blob/master/test/rocker_SUITE.erl#L743 but it's in a unused function (next_start_cf/1).

[liamaharon]

Hey @Elzor, thanks for your work here! Possible to review this? Thanks.