DEV: Ensure MF locales are checked properly

This patch fixes the `i18n:check` rake task which has been broken by
the `MessageFormat` upgrade.

It also adds a spec to ensure we generate valid MF code for all our
available locales.

spec/lib/js_locale_helper_spec.rb

@@ -144,6 +144,10 @@ RSpec.describe JsLocaleHelper do
         expect(content).to_not eq("")
       end
     end
+
+    it "generates valid MF locales for the '#{locale[:value]}' locale" do
+      expect(described_class.output_MF(locale[:value])).not_to match(/Failed to compile/)
+    end
   end
 
   describe ".output_MF" do