chore(package): update nyc to version 15.0.1

Closes #39

Author: greenkeeper[bot]

package.json

@@ -38,7 +38,7 @@
     "ava": "^2.0.0",
     "coveralls": "^3.0.3",
     "del": "^5.0.0",
-    "nyc": "^14.0.0",
+    "nyc": "^15.0.1",
     "standard": "^13.0.1",
     "standard-version": "^6.0.1"
   },