dependencies: update supertest 3.0.0 -> 4.0.2

This is a dev dependency. No impact in production. After this upgrade the
backend tests ("cd src; npm test") still pass.

src/package-lock.json

@@ -6687,13 +6687,13 @@
       }
     },
     "supertest": {
-      "version": "3.0.0",
-      "resolved": "https://registry.npmjs.org/supertest/-/supertest-3.0.0.tgz",
-      "integrity": "sha1-jUu2j9GDDuBwM7HFpamkAhyWUpY=",
+      "version": "4.0.2",
+      "resolved": "https://registry.npmjs.org/supertest/-/supertest-4.0.2.tgz",
+      "integrity": "sha512-1BAbvrOZsGA3YTCWqbmh14L0YEq0EGICX/nBnfkfVJn7SrxQV1I3pMYjSzG9y/7ZU2V9dWqyqk2POwxlb09duQ==",
       "dev": true,
       "requires": {
-        "methods": "~1.1.2",
-        "superagent": "^3.0.0"
+        "methods": "^1.1.2",
+        "superagent": "^3.8.3"
       }
     },
     "supports-color": {

src/package.json

@@ -70,7 +70,7 @@
   "devDependencies": {
     "mocha": "5.2.0",
     "nyc": "14.1.0",
-    "supertest": "3.0.0",
+    "supertest": "4.0.2",
     "wd": "1.11.4"
   },
   "engines": {