1. we now rely on $1's vendor.sh script (instead of duplicating it here)
2. we assume that hex_core is rebar3-compatible
3. we rely on TARGET_ERLANG_VERSION
4. we generate exactly what was previously generating, while targeting a specific Erlang/OTP vsn
We're planning to move the vendor.sh script from hex_core to hex,
but if you'd prefer it stays there it's not a problem. Thought by
moving it here I think it's a bit easier to use anyway.