chore(package): update nyc to version 15.1.0

Closes #42

Author: greenkeeper[bot]

package.json

@@ -77,7 +77,7 @@
     "eslint": "^5.0.0",
     "husky": "^1.0.0",
     "mocha": "^6.0.0",
-    "nyc": "^14.0.0",
+    "nyc": "^15.1.0",
     "rimraf": "^2.6.2"
   }
 }