Egg laying has nothing to do with the temperature, but rather the amount of light they're getting. If you want them to keep laying, set up a light in their coop on a timer to make sure they're getting about 14 hours of light a day.
They also don't lay when they're molting, so that also could be why they've stopped, as this is also the time of year that chickens molt.
Keep in mind that by forcing them to lay through the cold months you're not actually getting more eggs. A hen has a finite amount of eggs. Adding light just makes them keep laying instead of getting break, so they go through their supply of eggs faster and stop laying altogether sooner.