Add available_locales method to backend

This changeset adds an `available_locales` method to the `I18n::Backend:Http` base class. [I18n](https://github.com/svenfuchs/i18n) expects [this method](https://github.com/svenfuchs/i18n/blob/master/lib/i18n/backend/base.rb#L77-L81):

```rb
# Returns an array of locales for which translations are available
# ignoring the reserved translation meta data key :i18n.
def available_locales
  raise NotImplementedError
end
```