Evaluates Ruby on generating `en.yml` locale file

In order to evaluate the Ruby code `<%= app_name.humanize %>` in
`templates/config_locales_en.yml` introduced in 4d30479

* Change `config_locales_en.yml` to `config_locales_en.yml.erb`
* Use `template` when generating `config/locales/en.yml` file instead of
  copying the template file
* Add spec to ensure `en.titles.application` is set to App Name