chore(package): update nyc to version 15.0.0

Author: greenkeeper[bot]

package.json

@@ -16,7 +16,7 @@
   "devDependencies": {
     "ava": "^2.0.0",
     "codecov": "^3.0.0",
-    "nyc": "^14.0.0",
+    "nyc": "^15.0.0",
     "semantic-release": "^15.0.0",
     "xo": "^0.25.0"
   },