Remove deprecation warnings due to update of rails

* `config.serve_static_assets` has been renamed to `config.serve_static_files`
  and former will be removed in rails 5
* propagate errors raised within `after_rollback`/`after_commit` callbacks.
  Read more at: http://edgeguides.rubyonrails.org/upgrading_ruby_on_rails.html#error-handling-in-transaction-callbacks

config/application.rb

@@ -60,5 +60,9 @@ module Osem
 
     # Version of your assets, change this if you want to expire all your assets
     config.assets.version = '1.0'
+
+    # Errors raised within `after_rollback`/`after_commit` propagate normally
+    # like in other Active Record callbacks.
+    config.active_record.raise_in_transactional_callbacks = true
   end
 end