Add warning when TZ database version is changed

Changing the TZ database is useful for unit and compatability tests, but
production should rely on the current tested version in this lib. This adds
a warning message when a new version is supplied in the config to make that
point known

mix.exs

@@ -17,17 +17,7 @@ defmodule NervesTimeZones.MixProject do
       package: package(),
       source_url: @source_url,
       compilers: [:elixir_make | Mix.compilers()],
-      make_env: fn ->
-        tzdata_version = Application.get_env(@app, :version, @tzdata_version)
-        tzdata_earliest_date = Application.get_env(@app, :earliest_date, @tzdata_earliest_date)
-        tzdata_latest_date = Application.get_env(@app, :latest_date, @tzdata_latest_date)
-
-        %{
-          "TZDATA_VERSION" => tzdata_version,
-          "TZDATA_EARLIEST_DATE" => to_string(tzdata_earliest_date),
-          "TZDATA_LATEST_DATE" => to_string(tzdata_latest_date)
-        }
-      end,
+      make_env: &make_env/0,
       make_error_message: "",
       make_targets: ["all"],
       make_clean: ["clean"],
@@ -96,4 +86,26 @@ defmodule NervesTimeZones.MixProject do
       source_url: @source_url
     ]
   end
+
+  defp make_env() do
+    tzdata_version = Application.get_env(@app, :version, @tzdata_version)
+    tzdata_earliest_date = Application.get_env(@app, :earliest_date, @tzdata_earliest_date)
+    tzdata_latest_date = Application.get_env(@app, :latest_date, @tzdata_latest_date)
+
+    if tzdata_version != @tzdata_version do
+      msg = """
+      TZ database version #{tzdata_version} differs from the offically supported version #{@tzdata_version}
+
+      This is useful for unit and compatability tests, but should be avoided in production.
+      """
+
+      IO.warn(msg, [])
+    end
+
+    %{
+      "TZDATA_VERSION" => tzdata_version,
+      "TZDATA_EARLIEST_DATE" => to_string(tzdata_earliest_date),
+      "TZDATA_LATEST_DATE" => to_string(tzdata_latest_date)
+    }
+  end
 end