serve_static_assets has been deprecated in Rails 4.2
delete line
config.serve_static_assets = true
at
config/environments/production.rb and config/environments/test.rb
from
https://github.com/heroku/rails_serve_static_assets/issues/11
delete line
config.serve_static_assets = true
at
config/environments/production.rb and config/environments/test.rb
from
https://github.com/heroku/rails_serve_static_assets/issues/11