Fix deprication warnings from ruby and rails upgrades

[latazzajones]

DEPRECATION WARNING: The configuration option `config.serve_static_assets` has been
renamed to `config.serve_static_files` to clarify its role (it merely enables serving
everything in the `public` folder and is unrelated to the asset pipeline). The
`serve_static_assets` alias will be removed in Rails 5.0. Please migrate your
configuration files accordingly. (called from block in <top (required)> at
/tmp/build_a68dc3a7846b7e19c899a4dbb20b181b/config/environments/production.rb:23)