Add Rollbar to "Who Uses Dart"

Hi Dart team,

[Rollbar](https://rollbar.com) now plays nicely with Dart, thanks to [rollbar.dart](https://github.com/Mixbook/rollbar.dart) by @astashov. I saw some other services that work with Dart in this list, hence this PR. If that's not what this list is for, feel free to close this request.

Cheers,

Brian