fix: restore this when requiring timers (#644)

closes #614

src/flush-microtasks.js

@@ -15,7 +15,7 @@ try {
   const nodeRequire = module && module[requireString]
   // assuming we're in node, let's try to get node's
   // version of setImmediate, bypassing fake timers if any.
-  enqueueTask = nodeRequire('timers').setImmediate
+  enqueueTask = nodeRequire.call(module, 'timers').setImmediate
 } catch (_err) {
   // we're in a browser
   // we can't use regular timers because they may still be faked