chain-migrations: fix default.

Author: nodech

lib/blockchain/migrations.js

@@ -625,7 +625,7 @@ class ChainMigratorOptions {
     this.network = Network.primary;
     this.logger = Logger.global;
 
-    this.migrations = exports.migrations;
+    this.migrations = ChainMigrator.migrations;
     this.migrateFlag = -1;
 
     this.dbVersion = 0;