chore(package): update nyc to version 15.1.0

Closes #46

Author: greenkeeper[bot]

package.json

@@ -37,7 +37,7 @@
     "mocha": "^6.0.0",
     "mochawesome": "^3.0.2",
 
-    "nyc": "^13.2.0",
+    "nyc": "^15.1.0",
     "rollup": "^1.0.0",
     "rollup-plugin-node-resolve": "^5.0.0",
     "shelljs-release": "^0.3.0",