chore(package): update nyc to version 15.0.1

Closes #8

Author: greenkeeper[bot]

package.json

@@ -27,7 +27,7 @@
     "bindings": "^1.3.0",
     "mocha": "^5.0.0",
     "nan": "^2.8.0",
-    "nyc": "^12.0.1",
+    "nyc": "^15.0.1",
     "ref": "^1.3.5"
   }
 }