• Drop support to Rails 3 applications.
• Respect Rails.public_path when computing the path to the manifest file.
• Restore config.assets.manifest option, which was removed in Rails 4.0.0.
  This change does not affect existing behavior if the option is not set.
• Respect asset_host and relative_url_root to invalidate cache.
• Assets not in the precompile list can be checked for errors by setting
  config.assets.raise_runtime_errors = true in any environment