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/runtime.exs

@@ -0,0 +1,6 @@
+import Config
+
+config :wabanex, Wabanex.Repo,
+  username: System.get_env("POSTGRES_USER") || "postgres",
+  password: System.get_env("POSTGRES_PASS") || "postgres",
+  hostname: System.get_env("POSTGRES_HOST") || "localhost"