dependencies: update nyc 14.1.0 -> 15.0.0

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

This fixes 1 reported security vulnerability.

The output of `npm audit` goes from this:
  found 10 vulnerabilities (8 low, 2 high) in 13390 scanned packages
    run `npm audit fix` to fix 2 of them.
    1 vulnerability requires semver-major dependency updates.
    7 vulnerabilities require manual review. See the full report for details.

To this:
  found 9 vulnerabilities (7 low, 2 high) in 13707 scanned packages
    run `npm audit fix` to fix 1 of them.
    1 vulnerability requires semver-major dependency updates.
    7 vulnerabilities require manual review. See the full report for details.

src/package.json

@@ -69,7 +69,7 @@
   },
   "devDependencies": {
     "mocha": "5.2.0",
-    "nyc": "14.1.0",
+    "nyc": "15.0.0",
     "supertest": "4.0.2",
     "wd": "1.11.4"
   },