changed default EXTRUDE_MINTEMP to 170.

Marlin/Configuration.h

 //this prevents dangerous Extruder moves, i.e. if the temperature is under the limit
 //can be software-disabled for whatever purposes by
 #define PREVENT_DANGEROUS_EXTRUDE
-#define EXTRUDE_MINTEMP 190
+#define EXTRUDE_MINTEMP 170
 #define EXTRUDE_MAXLENGTH (X_MAX_LENGTH+Y_MAX_LENGTH) //prevent extrusion of very large distances.
 
 //===========================================================================