Bump @types/node from 12.20.13 to 15.6.0

Bumps [@types/node](https://github.com/DefinitelyTyped/DefinitelyTyped/tree/HEAD/types/node) from 12.20.13 to 15.6.0.
- [Release notes](https://github.com/DefinitelyTyped/DefinitelyTyped/releases)
- [Commits](https://github.com/DefinitelyTyped/DefinitelyTyped/commits/HEAD/types/node)

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

package.json

@@ -48,7 +48,7 @@
     "@types/bluebird": "^3.0.0",
     "@types/chai": "^4.0.0",
     "@types/mocha": "^7.0.0",
-    "@types/node": "^12.0.0",
+    "@types/node": "^15.6.0",
     "bluebird": "^3.0.0",
     "chai": "^4.0.0",
     "coveralls": "^3.0.0",