Fix missing promisify in disconnect.js, add flow

Author: BlooJeans

lib/modules/database/disconnect.js

@@ -1,5 +1,7 @@
+/* @flow */
 
 import { NativeModules } from 'react-native';
+import promisify from './../../utils/promisify';
 import Reference from './reference';
 
 const FirestackDatabase = NativeModules.FirestackDatabase;