Fix compile "typing" warning

We will check the config at runtime now instead of relying on it being
compiled as having the type set.

Fixes #157

lib/ecto/adapters/sqlite3/connection.ex

@@ -1384,11 +1384,9 @@ defmodule Ecto.Adapters.SQLite3.Connection do
     [quote_name(name)]
   end
 
-  @datetime_type Application.compile_env(:ecto_sqlite3, :datetime_type, :iso8601)
-
   defp expr({:datetime_add, _, [datetime, count, interval]}, sources, query) do
     format =
-      case @datetime_type do
+      case Application.get_env(:ecto_sqlite3, :datetime_type) do
         :text_datetime ->
           "%Y-%m-%d %H:%M:%f000Z"