Fixes issue with Fallbacks and enforce_available_locales

Fallbacks should check whether each fallback
location being tried was set by
store_translations before deferring to
I18n.translate. Otherwise a I18n::InvalidLocale
may be raised when enforce_available_locales is
set.

lib/i18n/backend/fallbacks.rb

@@ -40,6 +40,7 @@ def translate(locale, key, options = {})
 
         options[:fallback] = true
         I18n.fallbacks[locale].each do |fallback|
+          next unless translations.keys.include?(fallback)
           catch(:exception) do
             result = super(fallback, key, options)
             return result unless result.nil?