fix: load ActiveRecord component after Rails init

This isn't quite as neat as ea8bc34
(as I like the idea of waiting until ActiveRecord is loaded by Rails
itself), but it seems Rake tasks don't always load ActiveRecord, and so
we end up in a situation where indices are being loaded without TS's
ActiveRecord components being available (see
#1199).

But: the main issue was that prior to @jdelStrother's good work, AR
configuration in initialisers was being ignored. This new approach
waits until after initialisers have been run, so it should be fine.

Author: pat

lib/thinking_sphinx/railtie.rb

@@ -5,12 +5,12 @@ class ThinkingSphinx::Railtie < Rails::Railtie
     ThinkingSphinx::Configuration.reset
   end
 
-  initializer 'thinking_sphinx.initialisation' do
-    ActiveSupport.on_load(:active_record) do
-      require 'thinking_sphinx/active_record'
-      ActiveRecord::Base.include ThinkingSphinx::ActiveRecord::Base
-    end
+  config.after_initialize do
+    require 'thinking_sphinx/active_record'
+    ActiveRecord::Base.include ThinkingSphinx::ActiveRecord::Base
+  end
 
+  initializer 'thinking_sphinx.initialisation' do
     if ActiveSupport::VERSION::MAJOR > 5
       if Rails.application.config.autoloader == :zeitwerk
         ActiveSupport::Dependencies.autoload_paths.delete(