Update pins.h

Marlin/pins.h

   #include "pins_MEGATRONICS_2.h"
 #elif MB(MEGATRONICS_3)
   #include "pins_MEGATRONICS_3.h"
+#elif MB(MEGATRONICS_31)
+  #define MEGATRONICS_31
+  #include "pins_MEGATRONICS_3.h" 
 #elif MB(OMCA_A)
   #include "pins_OMCA_A.h"
 #elif MB(OMCA)