chore(package): update nyc to version 15.0.0

Closes #40

Author: greenkeeper[bot]

package.json

@@ -38,7 +38,7 @@
     "chai": "^4.2.0",
     "codecov": "^3.0.4",
     "mocha": "^5.2.0",
-    "nyc": "^13.3.0",
+    "nyc": "^15.0.0",
     "ts-node": "^7.0.1",
     "tslint": "^5.11.0",
     "tslint-config-standard": "^8.0.1",