chore(package): update nyc to version 15.0.1

Closes #46

Author: greenkeeper[bot]

package.json

@@ -28,7 +28,7 @@
   "devDependencies": {
     "ava": "^0.25.0",
     "coveralls": "^3.0.0",
-    "nyc": "^11.0.0",
+    "nyc": "^15.0.1",
     "snazzy": "^7.1.1",
     "standard": "^11.0.1"
   },