chore(package): update nyc to version 15.1.0

Closes #56

Author: greenkeeper[bot]

package.json

@@ -48,7 +48,7 @@
     "coveralls": "^3.0.2",
     "husky": "^3.0.0",
     "mocha": "^6.1.4",
-    "nyc": "^14.1.1",
+    "nyc": "^15.1.0",
     "standard": "^12.0.1"
   }
 }