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/dev.exs

@@ -1,11 +1,8 @@
-use Mix.Config
+import Config
 
 # Configure your database
 config :wabanex, Wabanex.Repo,
-  username: System.get_env("POSTGRES_USER") || "postgres",
-  password: System.get_env("POSTGRES_PASS") || "postgres",
   database: "wabanex_dev",
-  hostname: System.get_env("POSTGRES_HOST") || "localhost",
   show_sensitive_data_on_connection_error: true,
   pool_size: 10