Only remove spree_credit_card fields if they exist

I encountered a situation today on a database where these fields did not already exist. Best to have some kind of catch in place to not make the migration bomb out if the fields do not exist.

core/db/migrate/20131001013410_remove_unused_credit_card_fields.rb

@@ -1,12 +1,16 @@
 class RemoveUnusedCreditCardFields < ActiveRecord::Migration
   def up
-    remove_column :spree_credit_cards, :start_month
-    remove_column :spree_credit_cards, :start_year
-    remove_column :spree_credit_cards, :issue_number
+    remove_column :spree_credit_cards, :start_month if column_exists?(:spree_credit_cards, :start_month)
+    remove_column :spree_credit_cards, :start_year if column_exists?(:spree_credit_cards, :start_year)
+    remove_column :spree_credit_cards, :issue_number if column_exists?(:spree_credit_cards, :issue_number)
   end
   def down
     add_column :spree_credit_cards, :start_month,  :string
     add_column :spree_credit_cards, :start_year,   :string
     add_column :spree_credit_cards, :issue_number, :string
   end
+
+  def column_exists?(table, column)
+    ActiveRecord::Base.connection.column_exists?(table, column)
+  end
 end