Revert "Temporarily limit koa upstream tests to test against 2.15.3"

This reverts commit 95462ec.

Author: watson

packages/datadog-plugin-koa/test/suite.js

@@ -1,10 +1,7 @@
 'use strict'
 const suiteTest = require('../../dd-trace/test/plugins/suite')
 
-// TODO: Temporarily limiting this to run against v2.15.3 instead of `latest`,
-// as it's currently failing on `latest` because that code hasn't been pushed to GitHub.
-// For details, see: https://github.com/koajs/koa/issues/1857
-suiteTest('koa', 'koajs/koa', '2.15.3')
+suiteTest('koa', 'koajs/koa', 'latest')
 
 // TODO enable this
 // suiteTest('@koa/router', 'koajs/router', 'latest')