fix(package): update duplexify to version 4.1.1

Closes #2

Author: greenkeeper[bot]

package.json

@@ -30,6 +30,6 @@
     "testEnvironment": "node"
   },
   "dependencies": {
-    "duplexify": "^3.6.1"
+    "duplexify": "^4.1.1"
   }
 }