chore(package): update nyc to version 15.0.1

Closes #65

Author: greenkeeper[bot]

package.json

@@ -19,7 +19,7 @@
     "eslint-plugin-standard": "^3.0.1",
     "execa": "^0.10.0",
     "fixpack": "^2.3.1",
-    "nyc": "^11.0.1"
+    "nyc": "^15.0.1"
   },
   "directories": {
     "test": "test"