chore(package): update nyc to version 15.0.1

Closes #44

Author: greenkeeper[bot]

package.json

@@ -36,7 +36,7 @@
     "jsdom-global": "^3.0.2",
     "load-grunt-tasks": "^4.0.0",
     "mocha": "^6.0.0",
-    "nyc": "^14.0.0",
+    "nyc": "^15.0.1",
     "unexpected": "^11.0.1"
   },
   "babel": {