Update update_checker.py

Added connection close to header. Python 3.3 has this semi-dumb bug where the resource checker runs before the next 'line' of a program when you work with sockets. If you don't add a 'connection -> close' header, the socket stays open for a tiny period of time after you call it, and python throws a resource warning. The header makes the server close the socket when you are done with whatever you requested. It doesn't make a difference in real world but it prevents the resource warning from appearing.