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/environments/test.rb

@@ -8,7 +8,7 @@ Osem::Application.configure do
   config.cache_classes = true
 
   # Configure static asset server for tests with Cache-Control for performance
-  config.serve_static_assets = true
+  config.serve_static_files = true
   config.static_cache_control = 'public, max-age=3600'
 
   # Do not eager load code on boot.