chore(package): update nyc to version 15.0.0

Closes #2

Author: greenkeeper[bot]

package.json

@@ -17,7 +17,7 @@
   "license": "MIT",
   "devDependencies": {
     "codecov": "^3.2.0",
-    "nyc": "^13.3.0",
+    "nyc": "^15.0.0",
     "standard": "^12.0.1",
     "tap": "^12.6.1"
   },