Updates rebar3 to version 3.6.1, which amongst other things introduces
support for rebar3 on Erlang/OTP 21.
Changes made:
* rebar3 and dependencies updated to new versions
* rebar3 hermeticity patch updated to apply against new version
* hex package registry snapshot updated
This adds changes to the rebar3 expression that patch rebar3 to force it
to be hermetic. Now, by default, rebar3 literally can't download
anything. A 'rebar3-open' expression was added for those folks whe want
the normal rebar3.