Constantinople Fork Block (#382)

As per AllCoreDevs call #51 Constantinople fork is 7080000.

https://github.com/ethereum/pm/issues/64
https://github.com/ethereum/pm/blob/master/All%20Core%20Devs%20Meetings/Meeting%2051.md

config/src/main/resources/mainnet.json

@@ -9,6 +9,7 @@
     "eip155Block": 2675000,
     "eip158Block": 2675000,
     "byzantiumBlock": 4370000,
+    "constantinopleBlock": 7080000,
     "ethash": {
 
     }

ethereum/core/src/test/java/tech/pegasys/pantheon/ethereum/mainnet/MainnetProtocolScheduleTest.java

@@ -37,7 +37,9 @@ public class MainnetProtocolScheduleTest {
         .isEqualTo("TangerineWhistle");
     Assertions.assertThat(sched.getByBlockNumber(2_675_000L).getName()).isEqualTo("SpuriousDragon");
     Assertions.assertThat(sched.getByBlockNumber(4_730_000L).getName()).isEqualTo("Byzantium");
-    Assertions.assertThat(sched.getByBlockNumber(Long.MAX_VALUE).getName()).isEqualTo("Byzantium");
+    Assertions.assertThat(sched.getByBlockNumber(7_080_000L).getName()).isEqualTo("Constantinople");
+    Assertions.assertThat(sched.getByBlockNumber(Long.MAX_VALUE).getName())
+        .isEqualTo("Constantinople");
   }
 
   @Test