chore(package): update nyc to version 15.1.0

Closes #61

Author: greenkeeper[bot]

package.json

@@ -38,7 +38,7 @@
   "devDependencies": {
     "benchmark": "^2.1.0",
     "coveralls": "^3.0.10",
-    "nyc": "^14.1.1",
+    "nyc": "^15.1.0",
     "mocha": "^6.1.1",
     "standard": "^11.0.1"
   },