chore(deps): update dependency nyc to v15 (#925)

Author: renovate-bot

package.json

@@ -81,7 +81,7 @@
     "mv": "^2.1.1",
     "ncp": "^2.0.0",
     "nock": "^13.0.0",
-    "nyc": "^14.0.0",
+    "nyc": "^15.0.0",
     "proxyquire": "^2.0.0",
     "teeny-request": "^7.0.0",
     "tmp-promise": "^3.0.0",