Set I18n.config.enforce_available_locales to true to avoid an annoying deprecation warning

I was working on a plugin and after a bundle update and a I18n bump to 0.6.9 I started getting an annoying message about a deprecation warning:

```
$ be vagrant status
[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.
```

Looks like this was introduced on https://github.com/svenfuchs/i18n/pull/223 and an alternative would be to restrict the version on the gemspec.