chore(package): update supertest to version 4.0.2

Closes #17

Author: greenkeeper[bot]

package.json

@@ -28,6 +28,6 @@
   },
   "devDependencies": {
     "mocha": "^5.0.0",
-    "supertest": "^3.0.0"
+    "supertest": "^4.0.2"
   }
 }