chore(package): update nyc to version 15.0.0

Closes #325

Author: greenkeeper[bot]

package.json

@@ -58,7 +58,7 @@
     "json-stable-stringify": "^1.0.1",
     "mocha": "^5.2.0",
     "npm-run-all": "^4.1.5",
-    "nyc": "^13.1.0",
+    "nyc": "^15.0.0",
     "proxyquire": "^2.1.0",
     "sinon": "^7.2.2",
     "strip-ansi": "^5.0.0",