Fix service workers in production

assets/javascripts/app/config.coffee.erb

@@ -14,4 +14,4 @@ app.config =
   release: <%= Time.now.utc.httpdate.to_json %>
   mathml_stylesheet: '<%= App.cdn_origin %>/mathml.css'
   service_worker_path: '/service-worker.js'
-  service_worker_enabled: <%= App.environment == 'production' || ENV['ENABLE_SERVICE_WORKER'] == 'true' %>
+  service_worker_enabled: '<%= App.environment == 'production' || ENV['ENABLE_SERVICE_WORKER'] == 'true' %>' == 'true'