chore(package): update nyc to version 15.0.1

Closes #29

Author: greenkeeper[bot]

package.json

@@ -23,7 +23,7 @@
   },
   "devDependencies": {
     "coveralls": "^3.0.0",
-    "nyc": "^11.6.0",
+    "nyc": "^15.0.1",
     "standard": "^11.0.0",
     "tape": "^4.9.0"
   },