chore(package): update nyc to version 15.0.0

Author: greenkeeper[bot]

package.json

@@ -33,7 +33,7 @@
     "chai": "^4.2.0",
     "husky": "^3.0.0",
     "mocha": "^6.0.0",
-    "nyc": "^13.0.1",
+    "nyc": "^15.0.0",
     "source-map-support": "^0.5.9",
     "ts-node": "^8.1.1",
     "typedoc": "^0.15.0",