chore(package): update nyc to version 15.1.0

Closes #40

Author: greenkeeper[bot]

package.json

@@ -14,7 +14,7 @@
   },
   "devDependencies": {
     "nock": "^10.0.0",
-    "nyc": "^14.0.2",
+    "nyc": "^15.1.0",
     "proxyquire": "^2.0.0",
     "redis-mock": "^0.22.0",
     "standard": "^12.0.1",