chore(package): update nyc to version 15.0.0

Closes #25

Author: greenkeeper[bot]

package.json

@@ -35,7 +35,7 @@
     "eslint-plugin-prefer-let": "^1.0.1",
     "expect": "^24.1.0",
     "mocha": "^6.0.0",
-    "nyc": "^13.1.0",
+    "nyc": "^15.0.0",
     "rollup": "^1.0.0",
     "rollup-plugin-babel": "^4.2.0",
     "rollup-plugin-filesize": "^6.0.0",