Fix i18n warning about enforcing locale

This gets rid of the warning

```
[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.
```

See
http://stackoverflow.com/questions/20361428/rails-i18n-validation-deprecation-warning
Merge pull request #1487 from zhangsu/fix_i18n_warning

Fix i18n warning about enforcing locale