chore(package): update nyc to version 15.0.1

Closes #30

Author: greenkeeper[bot]

package.json

@@ -32,7 +32,7 @@
     "chai": "^4.1.2",
     "mkdirp": "^0.5.1",
     "mocha": "^5.0.0",
-    "nyc": "^12.0.1",
+    "nyc": "^15.0.1",
     "request": "^2.83.0",
     "rimraf": "^2.6.2",
     "ts-node": "^8.6.2",