update nyc to version 8.1.0 (#13)

Author: greenkeeperio-bot

package.json

@@ -21,7 +21,7 @@
   "devDependencies": {
     "ava": "^0.16.0",
     "coveralls": "2.x",
-    "nyc": "^7.0.0",
+    "nyc": "^8.1.0",
     "source-loader": "^0.2.0",
     "spike-core": "^0.11.0",
     "standard": "^7.1.2"