use short-form presence validator.

use two-argument form.
add unpaid scope.
repair tests.

spec/models/ticket_spec.rb

@@ -108,11 +108,11 @@ describe Ticket do
              user: user,
              ticket: ticket,
              quantity: 20)
-      expect(ticket.quantity_bought_by(user, :paid => false)).to eq(20)
+      expect(ticket.quantity_bought_by(user, paid: false)).to eq(20)
     end
 
     it 'returns zero if the user has not bought this ticket' do
-      expect(ticket.quantity_bought_by(user, :paid => false)).to eq(0)
+      expect(ticket.quantity_bought_by(user, paid: false)).to eq(0)
     end
   end
 
@@ -122,11 +122,11 @@ describe Ticket do
              user: user,
              ticket: ticket,
              quantity: 20)
-      expect(ticket.total_price(user, :paid => false)).to eq(Money.new(100000, 'USD'))
+      expect(ticket.total_price(user, paid: false)).to eq(Money.new(100000, 'USD'))
     end
 
     it 'returns zero if the user has not bought this ticket' do
-      expect(ticket.total_price(user, :paid => false)).to eq(Money.new(0, 'USD'))
+      expect(ticket.total_price(user, paid: false)).to eq(Money.new(0, 'USD'))
     end
   end
 
@@ -136,7 +136,7 @@ describe Ticket do
     describe 'user has bought' do
       context 'no tickets' do
         it 'returns zero' do
-          expect(Ticket.total_price(conference, user, :paid => false)).to eq(Money.new(0, 'USD'))
+          expect(Ticket.total_price(conference, user, paid: false)).to eq(Money.new(0, 'USD'))
         end
       end
 
@@ -146,7 +146,7 @@ describe Ticket do
         end
 
         it 'returns the correct total price' do
-          expect(Ticket.total_price(conference, user, :paid => false)).to eq(Money.new(100000, 'USD'))
+          expect(Ticket.total_price(conference, user, paid: false)).to eq(Money.new(100000, 'USD'))
         end
       end
 
@@ -158,7 +158,7 @@ describe Ticket do
 
         it 'returns the correct total price' do
           total_price = Money.new(200000, 'USD')
-          expect(Ticket.total_price(conference, user, :paid => false)).to eq(total_price)
+          expect(Ticket.total_price(conference, user, paid: false)).to eq(total_price)
         end
       end
     end