chore(package): update nyc to version 15.0.0

Closes #20

Author: greenkeeper[bot]

package.json

@@ -51,7 +51,7 @@
     "eslint-config-prettier": "^3.0.0",
     "eslint-plugin-prettier": "^2.6.2",
     "mocha": "^6.0.0",
-    "nyc": "^12.0.2",
+    "nyc": "^15.0.0",
     "prettier": "^1.14.0",
     "rimraf": "^2.6.1"
   },