Add a multi-column index on spree_reviews.approved + spree_reviews.product_id

Index these columns as they are used for scoping records
in the stock gem controllers (and likely used in any
custom controllers users are likely to write as well).

Author: nzaillian

db/migrate/20171004122538_index_product_id_and_approved.rb

@@ -0,0 +1,6 @@
+class IndexProductIdAndApproved < ActiveRecord::Migration[5.1]
+  def change
+    add_index :spree_reviews, :product_id
+    add_index :spree_reviews, :approved    
+  end
+end