chore(package): update nyc to version 15.0.0

Author: greenkeeper[bot]

package.json

@@ -33,7 +33,7 @@
     "conventional-changelog-jshint": "^2.0.0",
     "escape-string-regexp": "^2.0.0",
     "fs-extra": "^8.0.1",
-    "nyc": "^14.0.0",
+    "nyc": "^15.0.0",
     "proxyquire": "^2.1.0",
     "semantic-release": "^15.0.0",
     "sinon": "^8.0.0",