Fix deprecation `I18n.enforce_available_locales will default to true in the future ...`

This pull-request fix this deprecation message: 

```
[deprecated] I18n.enforce_available_locales will default to true in the future. 
If you really want to skip validation of your locale you can set I18n.enforce_available_locales = false
to avoid this message.
```

Gem uses Rails 3 as dependency, that is why this setting has been set to `false`.