* Upgrade elixir, erlang, and postgres to their latest versions.
* Fix CI configuration, so tests are executed.
* Migrate project configuration from `Mix.Config` to `Config`.
Reviewed-on: #5
To make easier to run the app locally with docker and in CI, the
configurations were adjusted to read the database access from
environment variables:
1. POSTGRES_USER
2. POSTGRES_PASS
3. POSTGRES_HOST
