A Sunwing Boeing 737-800, registration C-FLSW performing flight WG-803 from Toronto,ON to Vancouver,BC (Canada) with 188 people on board, was climbing through 10,000 feet out of Toronto when the crew noticed the cabin was also climbing through 10,000 feet. The Cabin Altitude Warning activated upon reaching 11,000 feet, the crew activated the related checklists and returned to Toronto for a safe but overweight landing on runway 23. Emergency services checked the brakes before the aircraft taxied to the apron. The Canadian TSB reported the aircraft had been dispatched unter minimum equipment list requirements with the #1 pack inoperative. The #2 pack became unserviceable during the climb due to a loose connector causing intermittent electrical power supply to the pack. The connector was secured and the aircraft returned to service.