Tests for ticket model

Change in error message of validation cause previously it was:
"Price currency Currency is different.."
`paid?` method returned true even when `ticket_purchases` is empty for
a given user.

app/models/ticket.rb

@@ -18,7 +18,7 @@ class Ticket < ActiveRecord::Base
   end
 
   def paid?(user)
-    ticket_purchases.where(user_id: user.id, paid: false).count == 0
+    ticket_purchases.find_by(user: user, paid: true).present?
   end
 
   def quantity_bought_by(user)
@@ -52,7 +52,7 @@ class Ticket < ActiveRecord::Base
 
   def tickets_of_conference_have_same_currency
     unless Ticket.where(conference_id: conference_id).all?{|t| t.price_currency == self.price_currency }
-      errors.add(:price_currency, 'Currency is different from the exist ticktes of this conference.')
+      errors.add(:price_currency, 'is different from the existing tickets of this conference.')
     end
   end
 end