Refactor - EventEmitter is already exported by 'events' module

The 'events' module already exports 'EventEmitter' constructor function - nodejs/node#2921

lib/application.js

@@ -7,7 +7,7 @@
 
 const isGeneratorFunction = require('is-generator-function');
 const debug = require('debug')('koa:application');
-const Emitter = require('events').EventEmitter;
+const Emitter = require('events');
 const onFinished = require('on-finished');
 const response = require('./response');
 const compose = require('koa-compose');