chore(package): update nyc to version 15.1.0

Closes #114

Author: greenkeeper[bot]

package.json

@@ -25,7 +25,7 @@
     "eslint-plugin-promise": "^4.0.1",
     "eslint-plugin-standard": "^4.0.0",
     "mocha": "^5.2.0",
-    "nyc": "^13.1.0",
+    "nyc": "^15.1.0",
     "postcss": "^7.0.7",
     "postcss-less": "^3.1.0",
     "postcss-reporter": "^6.0.1",