Use `config.serve_static_files` for Rails 4.2
Use `config.serve_static_files` for Rails 4.2

To suppress the following warning:
```
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.
```

bcavileer/ember-rails,bcavileer/ember-rails,maschwenk/ember-rails,bterkuile/ember-rails,emberjs/ember-rails,tricknotes/ember-rails,ipmobiletech/ember-rails,bterkuile/ember-rails,ipmobiletech/ember-rails,bcavileer/ember-rails,bterkuile/ember-rails,tricknotes/ember-rails,tricknotes/ember-rails,emberjs/ember-rails,emberjs/ember-rails,maschwenk/ember-rails,ipmobiletech/ember-rails,maschwenk/ember-rails