Make Translation.available_locales select distinct values.

Found on Rails 4.1.1. I haven't tested it on other Rails versions.

Before:
```ruby
> Translation.available_locales
SELECT DISTINCT `translations`.* FROM `translations`
```

After:
```
> Translation.distinct.pluck(:locale).map(&:to_sym)
SELECT DISTINCT `translations`.`locale` FROM `translations`
```