chore(package): update nyc to version 15.0.1

Closes #205

Author: greenkeeper[bot]

package.json

@@ -40,7 +40,7 @@
     "dfinity-radix-tree": "^0.2.5",
     "documentation": "^8.0.0",
     "level-browserify": "^1.1.2",
-    "nyc": "^12.0.2",
+    "nyc": "^15.0.1",
     "standard": "11.0.1",
     "tape": "^4.9.1"
   }