Bug fix: Asset pipeline was throwing can't modify immutable index.

Moving the `ActiveSupport.on_load(:after_initialize)` inside the `sprockets.environment` initializer wasn't enough. So we're checking if the `compile` flag is set, before registering the dependencies.

lib/i18n/js/engine.rb

@@ -10,6 +10,7 @@ class Engine < ::Rails::Engine
 
       ActiveSupport.on_load(:after_initialize, :yield => true) do
         next unless JS.has_asset_pipeline?
+        next unless Rails.configuration.assets.compile
 
         Rails.application.assets.register_preprocessor "application/javascript", :"i18n-js_dependencies" do |context, source|
           next source unless context.logical_path == "i18n/translations"