Bump firmware version to 1.3.1

Author: dihm

prawn_do/prawn_do.c

@@ -55,7 +55,7 @@ int status;
 #define EXTERNAL 1
 int clk_status = INTERNAL;
 unsigned short debug = 0;
-const char ver[6] = "1.3.0";
+const char ver[6] = "1.3.1";
 
 // Mutex for status
 static mutex_t status_mutex;