chore(package): update nyc to version 15.0.0

Closes #8

Author: greenkeeper[bot]

package.json

@@ -39,7 +39,7 @@
     "coveralls": "^3.0.0",
     "mocha": "^3.1.2",
     "nock": "^9.0.2",
-    "nyc": "^10.0.0",
+    "nyc": "^15.0.0",
     "standard": "^10.0.0",
     "standard-version": "^3.0.0"
   },