Update nyc to version 15.0.0 (#297)

Author: greenkeeper[bot]

package.json

@@ -36,7 +36,7 @@
     "coveralls": "^3.0.0",
     "eslint": "^5.0.0",
     "eslint-config-problems": "2.0.0",
-    "nyc": "^14.0.0",
+    "nyc": "^15.0.0",
     "postcss-import": "^12.0.0",
     "prettier": "~1.19.1",
     "sugarss": "^2.0.0",