Don't store translations for locales not set as available with I18n#available_locales=

If available locales are set explicitly, that means we don't want to work with other locales. This commit puts a check on Simple#store_translation to check if @@available_locales has been set explicitly, and in that case, does not store the data.

Right now, this translations are kept in memory and cached, but never used.

If the value of 'available_locales' is not set explicitly, it works as before, storing translations in any locale.