chore(package): update nyc to version 15.1.0

Closes #71

Author: greenkeeper[bot]

package.json

@@ -98,7 +98,7 @@
     "mocha": "^5.2.0",
     "mocha-reporter": "^0.1.1",
     "npm-run-all": "^4.1.3",
-    "nyc": "^13.0.1",
+    "nyc": "^15.1.0",
     "semantic-release": "^15.9.17",
     "travis-deploy-once": "^5.0.9",
     "ts-loader": "^5.2.1",