Suppress test.rb configuration warning for Rails 4.2.

Suppress following warning due to configuration changes.

```
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
/Users/Juan/dev/devise/test/rails_app/config/environments/test.rb:20)
```