feat: upgrade runtime and system dependencies (#5)

* 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

config/config.exs

@@ -5,7 +5,7 @@
 # is restricted to this project.
 
 # General application configuration
-use Mix.Config
+import Config
 
 config :wabanex,
   ecto_repos: [Wabanex.Repo]
@@ -32,4 +32,4 @@ config :phoenix, :json_library, Jason
 
 # Import environment specific config. This must remain at the bottom
 # of this file so it overrides the configuration defined above.
-import_config "#{Mix.env()}.exs"
+import_config "#{config_env()}.exs"