Add required plural form to locale

Transifex complained:

> Key 'spree.activerecord.models.spree/payment' has the wrong number of plurals

While our application may not need that translation, Transifex won't
accept the source file and our updates were broken.

config/locales/en.yml

@@ -3652,3 +3652,4 @@ See the %{link} to find out more about %{sitename}'s features and to start using
       models:
         spree/payment:
           one: Payment
+          other: Payments