use modern config handling

config/config.exs

@@ -1,6 +1,6 @@
 # This file is responsible for configuring your application
-# and its dependencies with the aid of the Mix.Config module.
-use Mix.Config
+# and its dependencies with the aid of the Config module.
+import Config
 
 # This configuration is loaded before any dependency and is restricted
 # to this project. If another project depends on this project, this
@@ -21,4 +21,4 @@ use Mix.Config
 #     config :logger, level: :info
 #
 
-import_config "#{Mix.env}.exs"
+import_config "#{config_env()}.exs"

config/dev.exs

@@ -1 +1 @@
-use Mix.Config
+import Config

config/prod.exs

@@ -1 +1 @@
-use Mix.Config
+import Config

config/test.exs

@@ -1,4 +1,4 @@
-use Mix.Config
+import Config
 
 config :holidefs,
   locales: [

lib/holidefs.ex

@@ -53,7 +53,7 @@ defmodule Holidefs do
     fed: "Federal Reserve"
   }
 
-  @locale_keys Application.get_env(:holidefs, :locales, Map.keys(@all_locales))
+  @locale_keys Application.compile_env(:holidefs, :locales, Map.keys(@all_locales))
   @locales Map.take(@all_locales, @locale_keys)
 
   @doc """

mix.exs

@@ -8,7 +8,7 @@ defmodule Holidefs.Mixfile do
     [
       app: :holidefs,
       version: @version,
-      elixir: "~> 1.5",
+      elixir: "~> 1.11",
       elixirc_paths: elixirc_paths(Mix.env()),
       package: package(),
       start_permanent: Mix.env() == :prod,