Elixir requires that we use version of the form `Major.Minor.Patch`.
Also, Mix requires dependecies to be declared in `rebar.config` so it
can include the header files correctly.
By treating the apps in the umbrella as dependencies to be managed and
built by rebar3, we can simplify the maintenance of the release, at
the cost of increased build times: using Mix as before, it could track
changed files better than using rebar. But the complexity and
possibility of discrepancies make it using rebar much more compelling.
