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/test.exs

@@ -1,4 +1,4 @@
-use Mix.Config
+import Config
 
 # Configure your database
 #
@@ -6,10 +6,7 @@ use Mix.Config
 # to provide built-in test partitioning in CI environment.
 # Run `mix help test` for more information.
 config :wabanex, Wabanex.Repo,
-  username: System.get_env("POSTGRES_USER") || "postgres",
-  password: System.get_env("POSTGRES_PASS") || "postgres",
   database: "wabanex_test#{System.get_env("MIX_TEST_PARTITION")}",
-  hostname: System.get_env("POSTGRES_HOST") || "localhost",
   pool: Ecto.Adapters.SQL.Sandbox
 
 # We don't run a server during test. If one is required,