chore(package): update nyc to version 15.1.0

Closes #54

Author: greenkeeper[bot]

package.json

@@ -53,7 +53,7 @@
     "mocha": "^5.0.2",
     "mocha-junit-reporter": "^1.23.1",
     "normalize-newline": "^3.0.0",
-    "nyc": "^11.4.1",
+    "nyc": "^15.1.0",
     "replace-ext": "^1.0.0",
     "rimraf": "^3.0.0",
     "standard": "^14.3.1"