Update supertest requirement from ^3.0.0 to ^4.0.0

Updates the requirements on [supertest](https://github.com/visionmedia/supertest) to permit the latest version.
- [Release notes](https://github.com/visionmedia/supertest/releases)
- [Commits](ladjs/supertest@v3.0.0...v4.0.0)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Author: dependabot-support

package.json

@@ -15,7 +15,7 @@
     "mocha": "^5.1.1",
     "nock": "^10.0.0",
     "sinon": "^7.2.3",
-    "supertest": "^3.0.0"
+    "supertest": "^4.0.0"
   },
   "scripts": {
     "test": "mocha"