Maybe you had already used
rails_12_factor gem in your project, if not, remember to add it in your Gemfile as :production group.
gem 'rails_12factor', group: :production
This gem enables serving assets in production and setting your logger to standard out, both of which are required to run a Rails 4 application on a twelve-factor provider. The gem also makes the appropriate changes for Rails 3 apps.
Maybe you want to attend a Deprecation Warning like:
DEPRECATION WARNING: The configuration option
config.serve_static_assets has been renamed to
config.serve_static_files to clarify its role.
Then you can update
But if after these changes you were not able to push to heroku, maybe is because you are having an issue in your css code or somewhere else.
rake assets:precompile on production environment.
RAILS_ENV=production bundle exec rake assets:precompile
And you can figure that out about your issue and receive feedback something like:
rake aborted! Sass::SyntaxError: Invalid CSS after "...ible !important": expected ";", was "!;"
Fix that issue or those issues, and there you go, you are ready to push to Heroku.