chore(package): update nyc to version 15.0.0

Closes #9

Author: greenkeeper[bot]

Diff for: package.json

@@ -14,7 +14,7 @@
     "chai": "^4.1.2",
     "mocha": "^5.0.4",
     "mocha-parallel-tests": "^2.0.0",
-    "nyc": "^11.3.0",
+    "nyc": "^15.0.0",
     "ts-node": "^6.0.0",
     "typescript": "^2.6.1"
   },