fix: Fixed sequelize-cli db:migration cannot stop when occur error

Up and Down method must return a Promise.

breaking changes: docker-hackmd cannot initialize, because db:migration will fail

lib/migrations/20161009040430-support-delete-note.js

@@ -1,10 +1,10 @@
 'use strict'
 module.exports = {
   up: function (queryInterface, Sequelize) {
-    queryInterface.addColumn('Notes', 'deletedAt', Sequelize.DATE)
+    return queryInterface.addColumn('Notes', 'deletedAt', Sequelize.DATE)
   },
 
   down: function (queryInterface, Sequelize) {
-    queryInterface.removeColumn('Notes', 'deletedAt')
+    return queryInterface.removeColumn('Notes', 'deletedAt')
   }
 }