Don't run sprockets code without sprockets

Author: jcoyne

lib/generators/blacklight_gallery/install_generator.rb

@@ -29,6 +29,9 @@ def add_openseadragon
 
     def assets
       copy_file "blacklight_gallery.css.scss", "app/assets/stylesheets/blacklight_gallery.css.scss"
+
+      return unless defined?(Sprockets)
+
       append_to_file 'app/assets/config/manifest.js', "\n//= link blacklight_gallery/manifest.js\n"
       copy_file "blacklight_gallery.js", "app/assets/javascripts/blacklight_gallery.js"