Update nyc to 15.x

Author: ankon

package.json

@@ -43,6 +43,6 @@
     "eslint": "^6.0.0",
     "eslint-config-collaborne": "^1.4.0",
     "mocha": "^6.0.0",
-    "nyc": "^14.0.0"
+    "nyc": "^15.0.0"
   }
 }