chore(package): update nyc to version 15.1.0

Closes #4

Author: greenkeeper[bot]

package.json

@@ -34,6 +34,6 @@
     "coveralls": "^3.0.2",
     "mocha": "^6.0.0",
     "mocha-lcov-reporter": "^1.3.0",
-    "nyc": "^13.2.0"
+    "nyc": "^15.1.0"
   }
 }