Update pins.h

Somehow the pin definitions for the MAX6675 Slave Select/Chip Select got changed back to the hardware SPI SS/CS.  
Pin 49 cannot be used as that is used by the SD card socket to detect if an SD card is present.
Pin 53 is the Hardware SPI SS, which is dedicated, nicely, to the SD card SS and therefore cannot be used as the MAX6675 SS
Pin 66 is in Aux2 port adjacent to the Hardware SPI interface port on RAMPS 1.4, and is currently not used for anything.