Reorder I18n backends in with_translations helper

The `with_translations` helper allows translations to be setup for a
method under test. A recent change (thoughtbot#772) added a backend chain to
ensure this helper won't break other spec by replacing all translations
and causing "missing_translations" failures.

This changes the order of the I18n backends so that specs will first use
the translations provided to the `with_translations` block and then
fallback to the default translations. Otherwise, there would be no way
to override the default translations in test cases.

spec/support/i18n.rb

@@ -7,7 +7,7 @@ def with_translations(locale, translations)
     new_backend = I18n::Backend::KeyValue.new({}, true)
     new_backend.store_translations(locale, translations)
 
-    I18n.backend = I18n::Backend::Chain.new(original_backend, new_backend)
+    I18n.backend = I18n::Backend::Chain.new(new_backend, original_backend)
 
     yield
   ensure