Ensure railtie initializer runs before app initializers

In some situations, this was running after the application initializers, which meant that config for custom fields was not taking effect.

This load-order behaviour seems to depend on what other Engines are installed, and the order they appear in the Gemfile.  In my case, it was the less-rails gem that triggered this behaviour.  If logstasher was declared before it in the Gemfile, everything worked as expected, but if logstasher came after less-rails in the Gemfile, the initializers order was changed, and the custom fields config did not run (because `LogStasher.enabled` was returning nil).