chore(package): update supertest to version 4.0.0

Author: greenkeeper[bot]

package.json

@@ -11,7 +11,7 @@
   "devDependencies": {
     "mocha": "^5.2.0",
     "semistandard": "^12.0.1",
-    "supertest": "^3.3.0"
+    "supertest": "^4.0.0"
   },
   "engines": {
     "node": ">=6.9.1",