Upgrade to rebar3

[kennedyjosh]

I'd like to upgrade lager to use rebar3 instead of rebar2.

There is a guide on some of the changes that would occur here: https://rebar3.readme.io/docs/from-rebar-2x-to-rebar3

Would this be a useful change?

[Vagabond]

Lager already works fine as a dep with rebar3 projects, no?

I don't have a strong objection but we might break some people who are stuck on rebar2.

[kennedyjosh]

You're right, it does work fine with rebar3 projects. I suppose I wrongly assumed it wouldn't.

We can maintain backwards compatibility for rebar2 while also supporting rebar3, though, so this is probably something that should be done sometime in the near future.