refactor to avoid node v6 issue where the process ends before process.stdout is flushed

example/mock-async/mocks/delayed.js

@@ -2,7 +2,7 @@ module.exports = {
   response: function (ctx) {
     return new Promise((resolve, reject) => {
       setTimeout(() => {
-        ctx.body = `<h1>You waited 2s for this</h1>`
+        ctx.body = '<h1>You waited 2s for this</h1>'
         resolve()
       }, 2000)
     })