Specify version of Erlang and rebar3 needed

[drozzy]

I think we need to set a specific version of Erlang as requirement, as I had some people run into issues when running old version of Erlang. I had the same problem myself (forgot to document which version it was). It might have been the combination of bad rebar3 version and erlang combined.

I recall 19.1 or 19.2 was not very good for some reason, can't remember why. Might be good idea to set 19.3 or even 20 as min requirement.
Similarly, figure out which rebar3 version we need (we do supply our own rebar3 binary - so make sure it matches the required erlang version)