Fix deprecation in engine mounting (#198)

Author: atomaka

lib/propshaft/railtie.rb

@@ -39,7 +39,7 @@ class Railtie < ::Rails::Railtie
 
       if config.assets.server
         app.routes.prepend do
-          mount app.assets.server => app.assets.config.prefix
+          mount app.assets.server, at: app.assets.config.prefix
         end
       end