add keys for all supported languages in I18n.translations.

previously only languages with translations on the page were
being added as keys to I18n.translations. this caused problems
in some cases where an empty translation file was being used
to send a language code to an external system.

test plan:
  * run rake i18n:generate_js;
  * open any page in canvas and, in a js console, examine
    I18n.translations;
  * verify that keys exist in the object for all languages in
    config/locales.

Change-Id: I64f5058dbfee5df0eacdd3a07374592e56523714
Reviewed-on: https://gerrit.instructure.com/12341
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>

lib/tasks/i18n.rake

@@ -226,6 +226,7 @@ namespace :i18n do
 
     dump_translations = lambda do |translation_name, translations|
       file = "public/javascripts/translations/#{translation_name}.js"
+      locales.each { |locale| translations[locale] ||= {} }
       content = <<-TRANSLATIONS
 // this file was auto-generated by rake i18n:generate_js.
 // you probably shouldn't edit it directly