chore(package): update nyc to version 15.1.0

Closes #7

Author: greenkeeper[bot]

package.json

@@ -23,7 +23,7 @@
     "chai": "^4.2.0",
     "coveralls": "^3.0.3",
     "mocha": "^6.1.4",
-    "nyc": "^14.1.1",
+    "nyc": "^15.1.0",
     "webpack": "^4.31.0",
     "webpack-cli": "^3.3.2"
   },