Bump nyc from 11.3.0 to 15.1.0

Bumps [nyc](https://github.com/istanbuljs/nyc) from 11.3.0 to 15.1.0.
- [Release notes](https://github.com/istanbuljs/nyc/releases)
- [Changelog](https://github.com/istanbuljs/nyc/blob/master/CHANGELOG.md)
- [Commits](istanbuljs/nyc@v11.3.0...v15.1.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

package.json

@@ -20,7 +20,7 @@
     "flow-copy-source": "^1.2.1",
     "mocha": "^4.0.1",
     "npm-run-all": "^4.1.5",
-    "nyc": "^11.3.0",
+    "nyc": "^15.1.0",
     "rollup": "^0.52.0",
     "rollup-plugin-babel": "^3.0.2",
     "snazzy": "^7.0.0",