message: Update MongoDB.md
  I think importing the dep makes thing a bit more clear. Some one might think that `NodeEnvironment` is a global variable
diff: @@ -42,6 +42,8 @@ Then we need a custom Test Environment for Mongo
  ```js
  // mongo-environment.js
  +const NodeEnvironment = require('jest-environment-node');
  +
  class MongoEnvironment extends NodeEnvironment {
  constructor(config) {
  super(config);
  