chore(package): update nyc to version 15.0.0

Closes #10

Author: greenkeeper[bot]

package.json

@@ -41,7 +41,7 @@
     "coveralls": "^3.0.1",
     "mocha": "^5.2.0",
     "moment-timezone": "^0.5.17",
-    "nyc": "^12.0.2",
+    "nyc": "^15.0.0",
     "prettier": "^1.13.0",
     "ts-node": "^6.0.3",
     "tsconfig-paths": "^3.3.2",