Bump nyc from 14.1.1 to 15.0.0 (#55)

Bumps [nyc](https://github.com/istanbuljs/nyc) from 14.1.1 to 15.0.0.
- [Release notes](https://github.com/istanbuljs/nyc/releases)
- [Changelog](https://github.com/istanbuljs/nyc/blob/master/CHANGELOG.md)
- [Commits](istanbuljs/nyc@v14.1.1...v15.0.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

package.json

@@ -64,7 +64,7 @@
     "jsdoc": "^3.6.3",
     "mocha": "^6.2.0",
     "np": "*",
-    "nyc": "^14.1.1",
+    "nyc": "^15.0.0",
     "should": "^13.2.3",
     "telegraf": "^2.0.0 || ^3.0.0"
   },