chore(package): update nyc to version 15.1.0

Closes #30

Author: greenkeeper[bot]

package.json

@@ -37,7 +37,7 @@
     "chai": "^4.2.0",
     "mkdirp": "^0.5.1",
     "mocha": "^5.0.4",
-    "nyc": "^12.0.1",
+    "nyc": "^15.1.0",
     "rimraf": "^2.6.2",
     "ts-node": "^8.6.2",
     "typescript": "^3.8.3"