Change `File.exists?` to `File.exist?`

* `File.exists?` is deprecated in favor of `File.exist?`

https://github.com/ruby/ruby/blob/trunk/file.c#L1482
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 4b0b8c1

* 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