change req sigs to 6 of 10

src/instantx.h

@@ -21,7 +21,7 @@
     ### getting 5 of 10 signatures w/ 1000 nodes of 2900
     (1000/2900.0)**5 = 0.004875397277841433
 */
-#define INSTANTX_SIGNATURES_REQUIRED           5
+#define INSTANTX_SIGNATURES_REQUIRED           6
 #define INSTANTX_SIGNATURES_TOTAL              10
 
 using namespace std;