Add filp/whoops dependency

As per [this doc](https://github.com/filp/whoops/blob/master/docs/Framework%20Integration.md#contributing-an-integration-with-a-framework), a depency on `filp/whoops` is required for an offical bridge.

I've recommended your package for slim. [see here](https://github.com/filp/whoops/issues/243).