chore(package): update nyc to version 8.1.0

https://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -54,7 +54,7 @@
     "chalk": "^1.1.2",
     "coveralls": "^2.11.8",
     "mocha": "^3.0.0",
-    "nyc": "^7.0.0",
+    "nyc": "^8.1.0",
     "standard": "^7.0.1",
     "standard-version": "^2.1.2"
   },