Update migration.js

Execute alterTable statements in order

Author: ataft

lib/migration.js

@@ -340,7 +340,7 @@ function mixinMigration(MsSQL) {
     statements = statements.concat(self.getDropColumns(model, actualFields));
     statements = statements.concat(self.addIndexes(model, actualIndexes));
 
-    async.each(statements, function(query, fn) {
+    async.eachSeries(statements, function(query, fn) {
       if (checkOnly) {
         fn(null, true, {statements: statements, query: query});
       } else {