core: add mainnet v6 fork height at 1400000

src/cryptonote_core/blockchain.cpp

@@ -99,6 +99,9 @@ static const struct {
   
   // version 5 starts from block 1288616, which is on or around the 15th of April, 2017. Fork time finalised on 2017-03-14.
   { 5, 1288616, 0, 1489520158 },  
+
+  // version 6 starts from block 1400000, which is on or around the 16th of September, 2017. Fork time finalised on 2017-08-18.
+  { 6, 1400000, 0, 1503046577 },
 };
 static const uint64_t mainnet_hard_fork_version_1_till = 1009826;