chore(package): update nyc to version 15.0.1

Closes #38

Author: greenkeeper[bot]

package.json

@@ -56,7 +56,7 @@
     "eslint-plugin-standard": "^4.0.1",
     "json-stable-stringify": "^1.0.1",
     "mocha": "^6.2.2",
-    "nyc": "^14.1.1",
+    "nyc": "^15.0.1",
     "stream-to-string": "^1.2.0",
     "thought": "^2.0.0",
     "thought-plugin-jsdoc": "^1.1.0",