chore(package): update nyc to version 15.0.0 (#46)

Closes #44

Co-authored-by: greenkeeper[bot] <23040076+greenkeeper[bot]@users.noreply.github.com>

Author: gucong3000

package.json

@@ -42,7 +42,7 @@
     "chai": "^4.1.2",
     "codecov": "^3.1.0",
     "mocha": "^5.2.0",
-    "nyc": "^13.0.1",
+    "nyc": "^15.0.0",
     "postcss": "^7.0.2",
     "postcss-safe-parser": "^4.0.1",
     "postcss-syntax": ">=0.34.0"